I assume this cannot affect the prioritisation of the Coinbase transaction that constitutes p2pool miner's reward payments? Can you clarify this?
The coinbase tx itself? No (it would be in the block you solve, nothing can change that).
Spending the outputs of the coinbase tx? Yes.
p2pool could implement BIP32 or in the interim given that p2pool nodes also run bitcoind could just implement a direct getnewaddress() RPC call after any successful block to ensure future blocks use a unique address.