While large pools have probably modified their clients anyway, it might be worth adding a -max_version command line option. It would default to INT_MAX.
If it is set, then only blocks with versions of 0 <= version <= max_version would be accepted.
This would mean that p2pool miners could easily update their nodes if the block version is updated.
If the code could be setup so that it only affects mining, then it wouldn't cause fork problems.
A user who only uses their client for transactions would be unaffected.
It's similar to adding the 1MB block size limit, and now we get stuck
Right, it would be critical that it is a "miner-only" patch, rather than an update to the general public's clients.
Another option would be to include updating the version number in the protocol.
If 750 of the last 1000 nodes include /VER_INC_BOTH/ in the coinbase, then the maximum version for transactions and blocks is increased by 1 from that point on. (/VER_INV_BLOCK/ and /VER_INV_TX/ could be used to increase one or the other).