I recently had one of my rims get really hot on a downhill. I had a problem with my front brake, and I had to use my rear brake only...
I did short, hard stabs instead of lightly dragging it.
I ended up walking much of the way once I found that my rear rim was too hot to touch.
I know that it was a worst-case-scenario, but I just want to confirm that I managed it the best way I could.
EDIT: To clarify, this is road-specific, not trails.