Failover, Wal Logging, and Multiple Spares

Assuming we are running a Postgres instance that is shipping log files to 2
or more warm spares, is there a way I can fail over to one of the spares,
and have the second spare start receiving updates from the new master
without missing a beat? I can live with losing the old master, and at least
at the moment it would be a controlled failover, but I would like to to know
if it's possible during an uncontrolled failover as well (catastrophic
hardware failure).
Right now, we have just that setup, but every time I've failed over to the
new master, we've had to rebuild our spares from scratch and unfortunately
this is a multi-hour long process. We can't afford the risk of not having a
warm spare for that length of time. We're planning to move entirely to a
slony cluster, but I'd like to fail over to a more powerful machine before
we begin the slony migration as the current server is already overloaded.

Thanks,
Bryan