You should be able to go a couple of miles slowly (under 40). If the battery state of charge gauge gets down to one or two bars and you aren't pulling into the station, it would probably be best to stop and bring gas to the car. Just keep it slow and keep some charge on the battery and you will be fine and not void your warranty.

My guess is that the bigger problem is emptying your fuel system and running your fuel pump dry, not damaging your battery. The fuel pump shouldn't be running while you are running electric, but it will need to reprime itself while dry after putting fuel in. That is, once you run out of gas, most of the damage to follow is probably unavoidable, save for running your batteries too low.

-- Alan