I see the processing equipment's value. Yet, it's really tantalizing to DIY it. Especially, when I've got 3 Optomas over here and they display a simple grid outline for a test pattern. The first thing I'd try is stacking them flat on top of each other and just using vertical shift, and hopefully that would be enough to sync. If none of the projectors need to be tilted at an angle, then the synchronizing of pixels should be pretty easy. If you've got to tilt one, then there's keystone correction to add into the mix, which might be too much futzing. And I think with either DIY setup or processing equipment, the mount has got to be really snug and jostle-resistant or you'd go crazy with the images going out of sync.
But that's all speculation. Maybe when I've got a little more time, I'll try it.