Not sure I understand the proposal. Is the coinbase a secret from the miners? How does the miner validate that the coinbase of the template they are mining on is not robbing them.
Does this scheme require a single coinbase output? This effectively rules out non-custodial mining unless we first activate CTV.
In general I think block withholding attacks are a minor problem, probably not worth a soft fork until it is demonstrated to be a significant problem.
Also, I think the solution is really simple and requires no consensus change at all: just pay a bonus for the block winning share.
Parasite Pool is immune to block withholding attacks because the block winning miner takes home 1 btc.
