Each p2pool node can use different version bits, so the final block version bits that you see depends on which p2pool node actually mined the block.
Are you planning to enforce BIP-91 for p2pool (assuming it locks in)?
If someone produces a share without the segwit bit set, then that share is effectively worthless (at least until segwit is locked in).
An easy rule would be that the bit must be set until August 1st and then while at least 750 of the last 1000 blocks have it set. That saves you needing to add code to figure out if segwit has been locked in.