[Dovecot] Request for variable unique to each server?

[Adam McDougall]

I've been looking for a variable I can use in my dovecot.conf within the INDEX=
setting so I can have one index dir per imap server on NFS.  I've been looking at
http://wiki.dovecot.org/Variables but no variable seems to contain something like
server hostname, and I've been hunting for the variable expansion code in dovecot
but haven't been able to find it.  %l (local IP) won't help me because when using
our load balancer, %l is the same IP on all of my servers.  %r is close to useful
but having a local webmail server on each server means I'd end up with 127.0.0.1 
being shared (also I would end up with one directory per unique user/clientIP combo).

Is there a variable that holds the server hostname or something else unique per server,
or could one be added, or could someone point me at the code?  Thanks.