Before I had an ATO I used to forget to top up my sump on my old 40B about 5 years ago all the time. It ran for 2 days once with very little water in it. Never seemed to have any effect on the pump or anything. I think you are being over-cautious. I don't think it is even possible for these style impeller pumps to catch fire. Although I suspect you could find
anecdotal evidence to the contrary

. We know the pump itself can run with the impeller not spinning (that is usually how these ultimately fail) and it doesn't do anything. Even if the impeller were to run totally dry I would suspect it would burn out and seize up before it created enough heat to cause a fire. And again, it is sitting in a pool of water. I feel more than comfortable with a Herbie setup. It is tried and true and again, I have never heard of a fire caused by a Herbie setup.
Maybe I will stick an old pump in a bucket, let the water evaporate out, and see what happens and how long it runs with no water.