p2pool is connected to the bitcoin P2P network and detects blocks before a getblocktemplate result is available for the new block and present a valid coinbase to miners ASAP. So there's absolutely no reason a "stale p2pool share" block would have more chance of being orphaned than any other block.
I assume getblocktemplate is used to build up the actual block transactions? So, there is a window where p2pool miners mine against a block that only contains coinbase?