Hi.

I'm struggling to get a mental picture of how this is all arranged and exactly which post hole has this issue. Could you take a picture of the top showing all the bridge and stop bar post holes, and mark up the offending hole?

You are certainly correct about the force on the stop bar and it's important that the inserts are securely held by the wood. The ply top alone won't be enough. But I don't have any idea how you'd get another piece of wood inside the body against the badly positioned centre block, let alone manage to clamp and glue it in place securely.

If it's a bridge post hole, almost all the force on that is downwards, so it won't pull out, though you'd probably have to epoxy the insert in place.

I'd be more tempted to look at alternative solutions like a trapeze tail piece, and block those post holes as best you can.

Another more drastic option would be to make it a 6-string by cutting down the headstock, fitting a new nut and bridge and adding a top mounted Bigsby-style trem unit which doesn't rely on the post holes but is mainly attached to the rear of the guitar.