[Exim] ldap lookup for whitelisting incoming mail

I'd like to somehow setup my exim to filter incoming email, such that if
the mail is from someone who is in my ldap address book, they get
whitelisted and dropped to a 'friends' folder on my imap server.

I've got exim to drop email normally into my imap server, and use a
~/.forward file to do simple per user filtering. However I got stuck
with this ldap lookup.

Since I have a nice ldap setup which is my address book (used by
evolution and mozilla), I thought it would be good to use this as my
whitelist.

I figured I couldn't do it in the .forward file (by piping out to
ldapsearch), since I won't get the results back until too late.

I then thought about about a special system router or transport that
does the lookup, and adds something to the header, which I could filter
for in the .forward files.

Anyone got any ideas, as to how the best way to do this is?

I've running Redhat Fedora 1, exim 4.30-5_3, dovecot imap 0.99.10-6 with
a Maildir format.

Arthur