[EXIM] Writing a director for arbitrary subdomains?

Currently, I have the following director which does smtp routing similar
to qmail:

d_smtproute:
  driver                = smartuser
  condition             = "${lookup{$domain}dbm\
                                {/var/exim/etc/smtproutes.db} {$value} fail}"
  transport             = t_smtproute

t_smtproute:
  driver                = smtp
  hosts                 = "${lookup{$domain}\
                                dbm{/var/exim/etc/smtproutes.db} {$value} fail}"

I use smtp routing to enforce that email to a particular domain is
transported by my system and everything works fine for the domain itself:

sub.domain: mx.host

Now it would be nice if the following would cause arbitrary subdomains
(like sub.sub.domain) to be handled:

.sub.domain: mx.host

A lookup option (similar to *@) that strips off subdomains one by one
until it finds a match would do the job. Is there an easier way than
hacking the source?

Michael

--
*** Exim information can be found at http://www.exim.org/ ***