[exim] Recipient Checking

I'm looking for a simple way to do recipient checking against my own
external daemon.

I have come up with a few possible way to do this:

1) Get queryprogram to simply query a unix socket instead?

     Problem) I doubt it supports this.

2) Run an smtp server so that exim could callout to it.

     Problem) SMTP and processing overhead.

  3) Add a lookup method to Exim to interact with a unix socket.
       Inventing some generic light weight protocol.

    Problems)
      1) Developing this feature into Exim.
      2) Ability to turn off caching since daemon allready does so?

So I'm wondering if there is a clear cut bulit in way for me to do this?
Or what my best route would be?

Thanks!

P.S.

The system allready clones saslauthd to centralize authentication for
Exim and Cryus daemons. Adding another unix socket server or smtp
server is not an issue as it allready supports such abilities... The
real issue is getting Exim to interact with the daemon.