Make Master arbiter for these new system tables – only the master can mod them – and then add a response on the heartbeat to update regionservers on last edit? Could be as simple as master just replying w/ timestamp of last edit.
We should do this as a part of (or using) HBASE-9864. I was thinking of something similar, where the data is kept in an hbase table as a snapshot + WAL. All transactions will have an trxid (NO timestamps please). All region servers open a session with a lease, and keep heartbeats to renew their lease. They send the last seen trxId, and the coordinator replies with the list of edits that they should apply to their in memory cache. If some reader looses it's leases, the coordinator (master) invalidates its session (so that there is an upper bound on the time the edits will be propogated). The coordinator keeps the last seen trxId per session, so that it can do recreate the snapshot and get rid of write ahead log entries.
However, astute readers might have noticed that this is indeed similar to zk's own protocol except that the data is not replicated via ZAB, but via datanode pipelines and hbase.