I have no experience w/ MFG, but I have done what you're asking in a UNIX environment with LaserJets.
If you know the escape sequence for the tray command (you can find these online usually), you can use PUT CONTROL to send the sequence at the very beginning of the print job. For a LaserJet 4 (I'm going by memory, so don't quote me), I believe the sequence was something like "&0L5" for tray # 2.
