[Exim] selective mysql relay

So I'm still having some troubles with this, but I also seem to always find
another possible "creative" solution.

Again, for those of you not following my ramblings -- I'm trying to create
an email relay that uses MySQL to implement a "whitelist" of sorts. Each
user has a "real" email and a "virtual" email, and a list of users they can
receive mail from.

Here is my question --

If I send 100% of email to an autoreply transport, can I then use SQL to
handle the "routing"? For example:
Process_Everything:
driver = autoreply
to = *some kind of SQL lookup that returns either the acceptible new to
address or /dev/null?*
reply_to = some other SQL lookup
subject = is there an expansion variable with the subject?
text = $message_body

My only questions -- is there an expansion variable that holds the message
body? (I did check the exim docs and I didn't seem to see one) Can you use
all expansion variables inside a transport?

-Erik Jacobs