[exim] Exim lsearch lookup in multiple files at the same tim…

I often need to have exim do a lookup in multiple files at the same time.
For example, to send a plain auth based on per-domain data:

PLAIN:
driver = plaintext
client_send = ${lookup{$sender_address}lsearch{/etc/exim/domain1}}


The way I solve it for now is to wrap a subsequent lookup into the negative
case of the preceding lookup like this:

  client_send = ${lookup{$sender_address}lsearch{/etc/exim/domain1}{$value}\
               {${lookup{$sender_address}lsearch{/etc/exim/domain2}}}}

This solution works, but it is hard to maintain (when the number of files
is large, it becomes very hard to keep track of the ever growing amount of
closing braces).

So my proposal is to allow a* list of files* as an argument to lsearch. Of
if there is an easier solution, I'm open to suggestions.

Regards,
Rustam