[Exim] expand fallback_hosts?

I have 4 mail servers. Local deliveries happen on one of the mail servers (NFS
server to the other three), so all the bounces get queued up on that server. I
would like to make the other three servers fallback_hosts for the NFS server
(but only for remote deliveries) so that remote bounces queue up on them.

I would like to keep a single Exim configuration on all 4 hosts and predicate
differences among them on the $primary_hostname. Can I force expansion of
fallback_hosts with double quotes or ${expand...}? I guess I could just add a
router if I have to.

The dnslookup router accepts "fallback_hosts" but not "hosts_randomize". Does
the smtp transport automatically randomize fallback hosts when called from
dnslookup?

I looked for the wishlist on the web site and in the exim-4.31/doc/ directory,
but I couldn't find it. Is this on the wishlist?
John