Off the wall, but have you also considered using Dovecot (or other software, eg nginx) proxy options?
I only have a small setup, but I have seen others use the idea that all backend servers are also frontend servers, but the proxy option then forwards the connection to the correct other machine (with only a few servers the probability they hit the correct server first time is also respectable).
You can also stick a single proxy only machine in front of everythingthing else to point connections to the correct backend straight off. Dovecot is apparently respectable as a high performance frontend proxy - some other people like nginx (no personal experience)
I believe you will find instructions on using indexes on NFS on either the wiki or a couple of previous answers from Timo here. I think basically that v1.1 is best for indexes on NFS?
Good luck
Ed W