No, I don't want to run it in the cold air return; but the return happens to
run precisely where I want the cable to run, across 8 joists. The return is
hung about an inch below the ceiling. Would it be considered adequate
protection to put the cable as close up against the return as I can get
staples into the ceiling?

I can't run the cable through the joists because of drain pipes and air
ducts, and I would rather not use a running board. I vaguely remember that
it is okay to run wire in the corner next to the wall, so I am hoping that
running it in the corner next to the air return would be similar. I could
just lay it above the air return, which would certainly be protected, but
then I couldn't staple it as required.