I'm reviving previous discussion.

#3 Index copy
this is what you are describing, copying the index using JGroups instead of my file system approach. This might have merits esp as we could diminish network traffic using multicast but it also require to rethink the master / slave modus operandi.
Today the master copy on a regular basis a clean index to a shared directory
On a regular basis, the slave go and copy the clean index from the shared directory.
In your approach, the master would send changes to the slaves and slaves would have to apply them "right away" (on their passive version)
I have been discussing with Manik on #3 and we think that JBoss Cache / Infinispan are probably a better fit than plain JGroups for that as all the plumbing will be configured for you.
When you reach this problem, let's revive this discussion.

I was thinking about possible ideas about this problem, but could I get some directions and advices where to start. Does JBC/Infinispan job should be to store index (kind of shared directory) copied from master, and makes it available for slaves? Or maybe better idea is to write custom DirectoryProvider based on JBC/Infinispan with passing over current master/slave directory providers?