Welcome (back) TPHB. I am not sure I would dump a gallon of heads into my fuel system. The problem with that, is there is a lot of water in the heads. It would depend upon your still, of course, more water if ur a pot stiller, less if from a column. However, ethanol and water mix, will separate when added to petrol fuel. Thus, what you are likely to get by doing this, is a load of water 'sludge' in the bottom of the gas tank. Not what I would like to have.
As for a fire, yes, it is 3 things, fuel/heat/O2 However, when you put water onto a ethanol fire, you have removed the fuel (and a lot of heat, btw). Low proof ethanol will not burn. Now, if the fire has spread, and started other things on fire, then you are 100% right, all bets are off, and water may NOT be the best thing. But for the home distiller, if you have a jar tip over or break, spreading hi proof on the floor which ignites, all you have to do is kick a 'at ready' 5 gallon bucket of water over, which also spreads across the floor, and the fire will douse immediately before it has a chance of setting the rest of everything on fire (hopefully). That is not to say you should not have a couple of well charged extinguishers available. I have 2 of them, one by each 'escape' exit, if I am not stilling outside. And that also 'assumes' you have a bucket of water (or 2) at the ready, and possibly also have a charged garden hose with a nozzle ready to go. Buckets full of water are the best bet, because when you kick them over you get ALL the water instantly, and you get it down under about 70 proof, and the fire will not sustain itself. 5 gallons of water (or better yet, 2 - 5 gallon buckets kicked over one after the other), will overwhelm a 1 gallon high proof accident every time, as long as you do so prior to other things being caught on fire.
Hillbilly Rebel: Unless you are one of the people on this site who are legalling distilling, keep a low profile, don't tell, don't sell.