Never done that myself to date - just inferred from BB demo files with midi guitar styles. Midi guitar is recorded assigning each string to a different channel. If that shouldn't be possible from within BB, PG Music must have used a different program such as a sequencer to record those midi bits and bytes. While that's possible, somehow it apears that BB would be lacking s.th. if it couldn't be done with BB means alone.

That's from the help file:

    Use Thru Channel: Band-in-a-Box uses the Thru channel as a part, just like the Bass/Drums/Piano, etc. The Thru Channel is re-channeled to the Thru channel as assigned in the MIDI settings dialog. If you would prefer to set the Thru channel yourself from your MIDI controller, set this to "No."

Sounds like unchecking this checkbox should enable the melody track to receive multi channel data sans rechanneling those data to some channel assigned in the Pref./Channels dialog.

Martin