What results might I expect if I used an A-T 849 stereo boundary mic mounted upside-down to a thin piece of plywood, then used that plywood to replace a ceiling tile in a suspended ceiling approx. 9 ft high? Would thick or thin plywood work better?

The shoot will be a presentation of a play, and every member of the cast & crew is challenged by some physical problem, (very much like stroke victims), and almost all have difficulty walking and must use aids (cane, wheelchair, etc). Thus, I can have absolutely no wiring on the floor for fear of someone tripping/falling. My plan at present is to obtain at least 2 wireless mics, but that will not be enough for the size of the cast. I need better sound overall. At present, the only other mics I have are 5 wired lavs and 2 shotguns.

Suggestions/recommendations invited.

If you don't already have the AT849, and are looking at purchasing something, I think I'd go with an AT hanging mic or two, like the U853R or RW (white). I think placement, coverage and safety would be much better all around with a hanging mic. I've done what you're asking before, but not with a boundary that large or with a cardioid pickup. Results were ok, but I think you could get better results with less setup effort with hanging mics.