It requires sending the entire block template (potentially up to 1 MB) to miners every 10 seconds, which is definitely impractical for remote miners. An extension to GBT that allowed only sending the merkle root would avoid this, though.
Sending only the merkle root, whether it be via getwork, Stratum, or some new GBT extension, makes the pool centralized, and kinda defeats the entire point of p2pool... Perhaps it would make sense for "remote" p2pool servers to be the first to move forward with mandatory-miner-provides-the-transactions GBT?