You can always drop down to the underlying PDO database connection and run whatever query you want. If table A is an ORMed table, though, and what you really want is to get your 3 A objects with id 1,4,5, then:

1) You should be able to use the ->where() method, but that one's new and looking at the code I don't believe it's actually implemented yet.

2) You could extend recess/recess/database/PdoDataSet to have an in method.

I'll look into this further later.