I can't say I've used GRDX or OMIT (or a U10M!) before, so excuse my ignorance please.
I assume the GRDX uses No of holes (or repetitions) in X and Y, and spacings between them. So I can't see how you can single out one pattern from within a larger pattern.
Maybe you could use separate GRDX commands either side of, above and below the ones you want to leave alone?
Sorry I can't be of real help, I thought you'd have had a ton of suggestions from the others by now.