Second the motion for a removable deck plate. Better yet, you might place a block on your coachroof to keep the tab from applying undue pressure on the deck.
If you want to stabilize the plywood against rot, there are a number of low viscosity epoxies out there that you can inject into the plywood. System Three, WEST, Smiths, and Boatlife come to mind. I've used GitRot (Boatlife) with good results. The trick is to wick it into the end grain of the wood, usually by drilling 1/8" hole--trying hard not to drill all the way through the wood--and injecting it with a squeeze bottle or syringe. I have used this technique on a non-structural bulkhead and on various wood house parts. Generally I remove any punky wood and apply the GitRot to sound wood. It has worked on a mitre joint in a wood gutter for over 20 years to arrest further rot. You can use a filler like polyester body filler (Bondo or similar) for any voids and then overcoat with gel coat for a finished repair on your coachroof.