HerHusband wrote:
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.

Actually, if the plywood patch is behind the plaster so you have the
full thickness to build up, you'll probably get better results if you
have some wallboard around in filling the bulk of the hole with it and
then finish it like taping a joint. A full thickness a plaster wall w/
mud is likely going to crack. If the ply is at or very near the
surface, as I was originally thinking, no sweat.