To update this discussion, I've meanwhile added a RELAY protocol to JGroups. This is issue JGRP-747 [1], which supercedes/replaces JGRP-105.
I've outlined the design of RELAY in a blog port [2]. In a nutshell, RELAY bridges 2 local clusters into a big virtual cluster. The local clusters are completely autonomous; it was paramount in the design to avoid a local cluster having to block on the other local cluster.
RELAY converts remote addresses to local ones and vice versa, and forwards (relays) traffic from a local cluster to the remote cluster. To be more specific, multicasts are forwarded (with the sender getting wrapped into a ProxyAddress) and unicasts are either sent locally (dest is not a remote address) or forwarded to the other cluster (dest is remote).
This works pretty well so far (as of Feb 2011), and even RPCs across the virtual cluster (with unicast replies) work. I'm currently working on making Infinispan work in DIST mode over RELAY.
I'll present this at JBossWorld 2011 (Boston in May).
[1] https://issues.jboss.org/browse/JGRP-747
[2] http://belaban.blogspot.com/2010/11/clustering-between-different-sites.html