Yes its quite easy
In Qlab set up a midi patch to IAC Bus 1
Set up some Midi cues in Qlab with program changes routed to this bus
In MainStage
set up some patches in the patch list. e.g IAC Bus 1 Ch1 Program number 3
In Edit Mode
Assign numbers to the patches in the Patch inspector attributes tab
In mainstage preferences/midi set midi program changes to 0...127 to get a 1 to 1 corellation between patch numbers
In Patch List highlight the concert name
In the concert inspector attributes set the program changes to IAC bus 1 ch1
That's it.
Mic