Check out Bayou (it's cited in the references of this paper, but not discussed). It's basically what you're describing: operations are initially assigned a temp

Check out Bayou (it's cited in the references of this paper, but not discussed). It's basically what you're describing: operations are initially assigned a temporary timestamp and distributed through gossip, and when they reach a central server, they're given a permanent monotonic sequence number that allows old entries to be garbage-collected.