Okay, I talked my roomate into letting me take apart his P90. The gearbox comes out quite easily, actually

The pictures make the sizes look closer than they are. The gearbox is taller than the receiver & top cover in real life (and when the top cover is properly aligned, which it isn't in these pics).
[img]http://www.vision-forge-graphics.com/jesse/props/images/1027_c.jpg[/img]
[img]http://www.vision-forge-graphics.com/jesse/props/images/1028_c.jpg[/img]
[img]http://www.vision-forge-graphics.com/jesse/props/images/1029_c.jpg[/img]
[img]http://www.vision-forge-graphics.com/jesse/props/images/1030_c.jpg[/img]
That's not to say it isn't possible though. The trigger box (that which encloses the real trigger on the prop and is connected by linkage rod to the rear clutch lever) is actually quite large, and easily wide enough to drop the gearbox down into. The forward length of the trigger box may have to be cheated forward somewhat, but otherwise I think we're okay.
Jesse
"Happiness is a warm smartgun."