So, I'm curious if I can apply joint compound directly over the
plywood patch? I would use fiberglass mesh tape to control cracking,
but will the compound stick to the plywood? The plywood is very dry
and has a rough texture.
The overall patch is about 6" wide x 9" high.

It will do fine. You could always staple a small chunk of screen or
similar mesh to provide a little more "teeth" but more than likely for
the purpose of a "temporary" patch no larger than that you could get
by with just a skim coat then a finish coat (or two).

I know it's practically impossible to remove joint compound I've dropped on
a plywood subfloor! So I figured it would work fine. I just wanted to
see what experience others have had.

In any case, I just want to pretty it up a little. The whole house could
use a complete gut and rebuild, but we can only do a little at a time.

Thanks for the feedback!

Anthony