They wouldn't need to resubmit to the main repo. They just fork it (like you did) include all the new development work done and paid for but leave out the little bit that makes the donation and then run the node. Presto - 0% fee. They could either release it publicly if they wanted to be an ass or better still just do it on the quiet and leach. It's just the existing system but without the convenient command line option that forrestv put in.

But the block chain concept already solved this problem. The same reason a miner can't just give themselves 50 BTC when they mine a block or double spend an input (analogous to removing a development charge). Although any miner could create a block awarding themselves extra BTC the rest of the bitcoin network agrees by consensus that this new block is not valid and simply ignores it - it doesn't enter the block chain. The rules of the system are encoded by the majority consensus of miners. That's the power of the 51% attack. With 51% you can change whatever rules you want and benefit yourself.

Taking this to the equivalent situation in the p2pool share chain. If the majority of miners on p2pool agree (or rather their nodes enforce) that a valid share must include a 1% donation to whomever then bang it's the law. Doesn't matter what an individual does with his nod. Nobody else will recognise it so he doesn't get that share. It's a hard fork though because you effectively create an entirely new p2pool which has an incompatible rule with the original p2pool.

So the technology is there to do something like that. But trouble is perhaps it's not really p2p anymore. You've just invented a semi-centralised pool that enforces a payment to the boss - even a benevolent one. So politically this could be hard to achieve in practice.