Heh - I was arguing that it should just work then got corrected (pointed out I was wrong) and worked out why back here:
https://bitcointalk.org/index.php?topic=18313.msg1311688#msg1311688

You now seem to be saying it will 'just' work

Implementing Stratum in P2Pool without adding the extra p2pool coinbase rules to the miner will be no different to using GetWork as far as I see.
Since without those rules, the only thing the miner can do is roll the nonce and the time.
So in reality it will just be 'getwork' not 'stratum' going to the miner.

You will still only be using roll-n-time in the miner until the miner knows the P2Pool rules to add to Stratum - since stratum updates the secondary nonce in the coinbase but of course doesn't update anything else in there, since for non-share chain, nothing else is required - but for p2pool you do have to change something for the share-chain (I've no idea exactly where it is in there)

However, I discovered yesterday that Stratum is flexible enough to roll any data in the coinbase transaction at all, though, so rolling something in the last txout script is probably possible (though it would require a change to P2Pool's rules).