The idea is really good and that is what I would like to do is create a sub pool and manage the payouts. There must be some sort of calculation that p2pool uses to determine which miner gets paid what. I just need that same information from p2pool.
p2pool pays using PPLNS, but unlike a normal pool where you submit small bits of work many times per minute and those shares are all stored and added up, p2pool uses a "sharechain" which is the same thing as bitcoin's block chain but at a much lower difficulty. To get paid in p2pool, you need to find a new valid block for the sharechain. Payouts are based on the work you have stored in the sharechain. This way you don't have to trust the other nodes people are running, there is no way for them to cheat the system, since they have to find valid sharechain blocks to get paid at all and that is just as reliable as bitcoint's blockchain itself.
For example, a normal pool might give you diff 128 work and you find 10 shares per minute on average. So every minute you are earning 1280 diff-1 shares of value for the PPLNS window to base your payouts on. But p2pool might have a minimum share difficulty of 128000. So you'd find 1 valid share every 100 minutes on average. Until you find that share, you do not get any payouts because your value in the PPLNS window is 0. Once you get a share, you get paid on all blocks found while your share is still in the window.
What you want to do requires the proxy system I linked before or some custom solution you come up with. The closest you could get using p2pool's own info would be to look at the pseudo share difficulty graphs for the miners (ie: their hash rate history) and try to do payments based off that somehow, if you don't want the proxy to store all of the tiny shares directly.