My guess would be that you should be fine. My subaru almost passed inspection with an aftermarket exhaust and an obvious aftermarket downpipe. It also had an intercooler added to it that wasn't there when it was stock. They only complained about the downpipe. The other stuff they completely ignored. Now, my subaru is a 91, so it's a bit old for them to be up on what should and shouldn't be there.
With something so minor as your deckplate, I think you should be just fine.