How to add new key lookup type?

Hello --

We'd like to use exim's from-address rewriting, only instead of looking up
the addresses in a plain text or dbm file, we'd like exim to call an
external program which would accept a from address as an arg and return a
new from address on stdout. (Our ultimate goal is to lookup the name in
a sql table, but I think making a small shell script driver like I have
described would be easier than making a full interface to our database.)
Is there an easy way to do something like this? If not, is there a hard
way? =)

Similarly, would it be reasonably easy to make a director driver that
would call an external program to determine whether or not an address
could be handled by that director?

Thanks.

-Chris