[Exim] $interface_host ?

Is there any easy way to get the (reverse DNS) canonical hostname
associated with $interface_address ? It would make it much easier
for me to set up certain aspects of IP-based virtual domains.

I know I could set up a file and use lookups to do the translation;
but it would be much cleaner if I could get it from the existing
DNS database.

If there is no easy built-in method at this time, I'd like to
propose either:

    1)    Add an $interface_host variable that would be set to the
    reverse DNS lookup of $interface_address.  (When that
    variable is valid.)

Or
    2)  Add a general DNS lookup capability that would take a key
    and a record type.  (With some magic to recognize and convert
    IP address keys into the in-addr.arpa form when PTR records
    are requested.)

Thanks,
-Pat