John- some time ago you left a comment in my Rejuvenation journal about camping reservations in Yellowstone.  The comment was apparently attached to an image that I've subsequently removed and replaced, so your remarks are now lost except in the Comment history in my journal: there's no way to Reply directly to it any longer.  I'm starting this thread because I haven't any other means of reaching out to you.

I was just doing some tidying up of my itinerary details and discovered that Bridge Bay CG (on my itinerary) was nearly fully booked up; in fact, I snagged THE LAST RV/Tent site reservation to be had, for the date I plan to be there.

HOWEVER: there seems to be an unpublicized "hiker/biker" accommodation as well, for $7.75/night rather than the $29 I'm paying for a "normal" campsite.  It may be that you can still get those, if you need them, but it may also require a telephone call to Xanterra (the concessionaire that runs reservation services for several of the most popular National Park Service parks).  "We are currently experiencing very high call volumes", says the warning banner on the website, so expect some frustrating wait times if you drop the dime.

To make this thread of greater general interest, I'll ask this question of the larger community: who has experience using (and booking?) the hiker biker campsites in the large, popular U.S. parks?  Are advance reservations needed, or is there a general "no rider turned away" policy in place, such that one could reasonably expect to have *some* accommodation even when showing up unannounced and unheralded?

I can comment on Yellowstone and Glacier National Parks. In the campgrounds I stayed in, no reservations were needed - I don't think you could make a reservation even if you wanted to do so - and there is a no-turn-away policy for cyclists. Click on my name and check out the Transam (for Yellowstone) and Northern Tier (for Glacier) journals for details if you desire. You should be aware that some state parks also have a no-turn-away policy for cyclists - Whitefish state park, Montana is an example.

Thanks, George!  I'm currently booked into the Canyon (or Canyon Village?) campground for one night, and the Bridge Bay campground for the next.  I'll check your TransAm journal to see whether you mention the specific campgrounds where you stayed.

Making a reservation on the Xanterra-operated Yellowstone website is a real adventure, in and of itself, and there's only the briefest of mentions of the hiker/biker accommodations with no further detail that I could unearth so it's good to be able to tap first-hand knowledge.

The north rim of Grand Canyon National Park has a large backpacker campground conveniently separate from the lodge and drive-up campground, but conveniently close to the grocery store. Traveling cyclists also use the campground. The campground has space for dozens of tents but doesn't have designated sites or tent pads. My site had a view into the canyon. The North Rim opens on May 15 every year.

The south rim has a similar number of backpackers starting or finishing rim-to-rim hikes. The Mather campground at the south rim has a smaller designated hiker/biker area. Found online:

Hiker/biker sites 24A-F and 25A-F are communal/shared sites and hold up to 1 person, 1 tent, NO vehicles and NO parking available for these sites.

At the east end of the Grand Canyon south rim, Desert View campground does not have a hiker/biker site because it has no major canyon trailheads. You can reserve regular campsites at Desert View, and your odds of showing up and finding an available site are good during weekdays.

Grand Teton National Park also has a hiker/biker site at every campground. The beautiful tents-only Jenny Lake campground has a car camping loop and an adjacent 12-site hiker/biker campground.

As others have stated, every campground at Yellowstone National Park has a communal hiker/biker area except for the campgrounds that don't allow tent camping because of bear problems.

Cougar Rock Campground in Mt. Rainier National Park has a hiker-biker camping area, I think with a no-turn-away policy. But when I stayed there a few years ago, I couldn't find anything about a hiker-biker site on the NPS pages, so I reserved a regular site. So I think the lesson is that if a campground's web site doesn't say it has a hiker-biker site, it still might, so call or email them (or do some serious googling, there are lots of campground review sites that might mention it).

Re Grand Canyon South Rim Desert View campground:

This is the handiest campground if you make the long climb up from Cameron. As you note, no hiker/biker site. The other sites at Desert View fill up fast. Early in season we arrived there by bikes without reservation, hoping to be early enough to grab a site. Didn't work. The Canadian couple on motorcycles right in front of us got the very last "overflow" site and we were SOL. I offered to pay for the site if they would allow us to share (up to four campers per site, I think). They agreed. We actually pitched tents outside the formal boundary of the campground, so they really had the site to themselves, and we shared the night with wandering elk.

https://www.cycleblaze.com/journals/gc2015/

I think the lesson is that if a campground's web site doesn't say it has a hiker-biker site, it still might

Mazama campground at Crater Lake National Park has a relatively new  hiker/biker  communal site that is not well known. It didn't exist the two times I camped there.

Just north of Crater Lake, few cyclists know about the free 5-site hiker/biker campground on the south shore of Diamond Lake. It's only accessible by a bike trail, in a spectacular setting between two volcanoes.

Silver Lake to Diamond Lake

My journals have evolved since 2006. I am disappointed that I did not document in detail the HOARDS of mosquitoes that covered our tent at Wayne's afore mentioned hiker biker site on Diamond Lake. It was like something out of a nature horror movie. Our tent fly was black with humming mosquitoes, just waiting for fresh blood! 

I remember, but did not document, a man Wayne knew who worked at the lodge, saying that the mosquitos had just hatched. Our timing was off. The photos with all of that snow were memorable.

I stayed at the hiker-biker sites at Mazama in 2017. At that time, the "sites" were just a cluster of small clearings linked by a network of unofficial trails in some buggy woods--no picnic tables, I think just one fire ring. It was full of Pacific Crest Trail through-hikers who stayed up late talking and playing guitar. As an old curmudgeon who likes to get to sleep early, I would have rather stayed in a regular campsite. The hiker-biker sites on the North Rim of the Grand Canyon, in contrast, had multiple picnic tables and an amazing view, and had enough room that it would have been easy to keep to my curmudgeonly self if there had been anyone else there. 

Some Forest Service campgrounds have hiker-biker sites too. This one, on Timothy Lake in Oregon, is my favorite so far: