Although I have never seen such a helpful policy in writing, I have benefitted often from NP rangers offering options when campgrounds are full. They may suggest a walk-in campsite, sharing a site with others, or turn a blind eye after mentioning a closed campground undergoing repair or an area that goes unpatrolled.
They are stuck with some rigid rules and limited discretion most of the time yet empathize with bike tourers stuck with few options other than a long ride in the dark.
Anyone with direct knowledge of such a policy in writing?