This would also be obvious to miners (if they know what to look for) and there would be no reason for them to pay a pool fee if they are just sending the work to p2pool.
I think there's no shame in it. If a user doesn't want to join p2pool himself it's probably because he doesn't know how to set it up, or doesn't have resources to maintain a full copy of the blockchain. In any case, whatever reason he has to prefer any particular pool, this reason still remains valid.
There have been some pools that frontended p2pool in the past, some haven't advertised it loudly (you can notice them by observing that their hashrate is the same as p2pool's).
cool. (Except it screws the hashrate statistics.) I wonder why isn't it more common then. May be it'd be good to convince one of the bigger pools doing it (like ~10% hashrate), may be then smaller ones would flock in and multiply.