Well, in the past I have use a “start message” that gets sent to BizTalk from a scheduled task or windows service. Then, I put my code after the receive shape inside an expression shape. The start message doesn’t do anything but start the Orchestration.
It might make sense to look at the community ODBC adapter on gotdotnet. Or even write your own customer adapter to read from the database (this will take some time though).
Hope this helps.