Using CMS, any chance of forewarning a serial full GC is imminent?

[Stack]

Our app, a distributed database, usually does fine running CMS but
when we trip a full serial GC, its disruptive (speaking
euphemistically).

Monitoring the running application, watching the logs or patching into
the JVM-TI, is there any indicator that you know of that would give us
forewarning of an imminent full serial GC?  If we had this, we could
take evasive action.

Thanks,
St.Ack

P.S. G1 is what we really need but going by the response up on this
and by how easy our application crashes recent releases, it looks like
its going to be a good while before it'll work for our case (We're an
open source database so talking to our sun/oracle vendor is not an
option).