[exim] Custom Variable

Well, I have been scratching my head on this for the last 4 hours, and it is
time to ask for help. I have scoured google, the archives, and the exim
docs to no avail. But, it is probably something super simple I am just not
seeing.

I need to assign a custom variable from data in a MySql lookup for use in a
transport. Like this:

my_transport:
      driver = pipe
??--> ryans_var = ${lookup mysql{select blah from blah where
column='$local_part'}{$value}}
      command = "/usr/local/bin/prog --option $ryans_var"

I just can't figure out how this is done. Exim pukes on anything I try. I
have tried using:

environment = "ryans_var = ${lookup mysql{select blah from blah where
blah}{$value}}"

Exim doesn't puke with that entry, but it does bounce the email saying
ryans_var doesn't exist.

Any ideas out there?

Thank you,

Ryan