I don't think it's ever a good idea to run a pump dry. Have you thought about making an autotop off using an air pump.
Check out the one Kwirky made....
http://www.canreef.com/vbulletin/sho...t=32423&page=2
I built one using a plastic water jug and it works like a charm. If it runs dry it just blows air... No worry about ruining a pump or worse starting a fire.... And it's cheap, I made mine with supplies I already had laying around.