https://e621.net/wiki/show?title=cock_hanging is empty and there are a variety of posts tagged with it, certainly some that will not fit under the same definition unless it is broad, probably too broad.
Personally, I would define it as a character being mostly supported by the penis of another or by their own penis inside another character, which would cover/include all knot_hanging posts but could apply to a slightly wider range.
There is no knot is this post https://e621.net/post/show/1618817/ but the character being penetrated is being supported by three things: The locker (tagged as against_wall), his tail (added tail_grab, tail_pull), and the other character's penis. I'm not sure if that should count as the penis isn't providing the majority of support, merely a third, but I'll leave that to discussion.
Looking through cock_hanging I'm finding a number of pieces mistagged as that instead of piledriver_position, cleaning those up. This one: https://e621.net/post/show/1495026/ isn't piledriver, but I'm not sure if it should count as cock hanging, will leave it for now.
Enough of my rambling, opening the floor.