If the ply is at or very near the surface,
as I was originally thinking, no sweat.

Yep, the plywood patch is screwed over the existing lath, leaving the
surface of the ply less than an 1/8" lower than the surrounding plaster.

Normally I would patch in with sheetrock, but when you're in a bind, you
use whatever is available...

Sounds like the joint compound should work well. Thanks!

Anthony