[exim] re-use looked up data in a router

Hi,

I've got a redirect router like this halfway my config, to redirect a received
mail to another separate address:

pipes_forward:
   driver                     = redirect
   domains                    = +pipe_domains
   data                       = ${lookup mysql{\
               SELECT `forward_copy` \
               FROM `pipes` \
               WHERE `domainname`='${quote_mysql:$domain}' \
               AND (`user`='*' OR `user`='${quote_mysql:$local_part}') \
               AND NOT `forward_copy` IS NULL} \
                                 {$value}fail}
   unseen

which works quite OK. I'd like to add the address that I've just queried to
the 'To' header.

I thought I could do this with:

headers_remove = To\nDKIM-Signature
headers_add = To: $header_to:


But I'm stumped for a variable that has the value from the query in it and I'd
like to not do the query again. I'm reading the documentation as we speak but
as you hopefully can imagine 'data' is not a very useful word to be searching for.

Is there a way?

thanks,
Maarten.