I wanted to do something similar, but I didn't want to hunt down a out-of-production switcher to do it... so I just got a Switchblade 8B. It's more versatile, programable, but also costs more.
I use it to combine my 2 Mark III heads, triaxis-2:90, Marshall 9001, and G-major into one guitar rig, with a 4x12 center cab and two thieles on either side in stereo.
I think the only way you could do that with the Mesa switcher would be to put the g-major after the output of the switcher, but then all of your sound would always go through the g-major in series... :?
edit - added linky: http://www.soundsculpture.com/products/switchblade_features.htm