It is already implemented and available in the latest Infinispan 5.2 CRs.
As for ISPNs, there is a "Cross-site replication" component in Infinispan's Jira.
If I try using the xml files attached to https://docs.jboss.org/author/display/ISPN/Cross+site+replication#Crosssitereplication-Reference (with the changes required for our application), I get exceptions in parsing the xml on the global configuration for sites. It's as if it is not recognising the sites element as valid (Unexpected element).
I am using the latest infinispan (5.2.0cr3).
After looking through the xsd it appears that the xml files/examples given in the documentation are now out of date and so cause errors.
@bevans88 - thanks for reporting this, the documentation should be now fixed.