This was my thought as well. Don't know if you're using a control system of any kind but pretty much any commercial projector worth a damn has rs232 or ethernet control, you could even run a single command from hyperterm or similar. With most commercial projectors you can even address them so you could feed once daisy chained serial cable to all of them and only the unite with the address you specify will respond to the unmute command.
Ethernet commands are very close to the same idea but I don't think you can use hyper term? maybe just a commanf prompt I don't know but the rs232 version would be easy for sure.
I guess this comes down to the importance of the show again, but in most cases I would rather see the image get dull than have the entire image drop out until it could be switched over. Then again double stacking projectors is very familiar to me and I use projectors that can do this well so correct alignment is less of a consideration.
As a whole system it is quite rare to have a system that has full redundancy - even with control and playback backups unless you have multiple inputs going to your display devices you will still have a matrix or similar to do the switchover unless you want to have to transfer over to an entirely new system if you have a fault. I have even had a projector fault in a way that it was flashing the image continuously and the whole control system was jammed from the same issue - the only solution here was to pull the power - thankfully this was pre show, as no control solution would have helped here.
Situations like that encourage the idea of having 100% separate power and signal flow to the individual projectors that terminate to a place that you have easy access to. This is not always possible, but I do it wherever I can.
-Gareth
