I'd suggest you ask a local official for your codes on that. I see no reason to not do what you plan; if wood cabinets are OK then a sheet of ply should be. But, that is just a guess; see an official. Could you live w/ the plywood covered w/ sheet rock and perhaps cut a tad undersize so the sheet rock would cover it and you could mud the edges? If you will be welding or have open flames, I'd sure sheet rock over the ply, code or not.