Performing a series of positive filters on incoming mail..

Suppose that I could get spamassassin to add a header that states that 
the email passed domain keys verification.

Is there any way that I could then have tmda somehow do:

Is the from address domain <SOME DOMAIN SPECIFICATION>?
No? Fall through to rest of incomng filters.
Yes, Is the header that spamassassin added present?
No? Drop the mail.
Yes? Deliver to maildir.

This would help block spoof of ebay, google, or paypal (which all seem 
to be sending with domain key signatures.)

I wondered if using the pipe source might be the way to go, but I would 
have to do a lot of the header processing myself.  Is there 
documentation on the tmda ABI?

Thank you,
Tony