The intent of the ash temperature probe as it is now is to make the Coal Gun easier to use by eliminating the need to frequently adjust the grate timer as the heating season progresses and more fuel efficient by eliminating any partially burnt coal in the ash. So far, I haven't heard of anybody doing what you propose with the thermocouple setup, but it is an intriguing idea.
First, I have to ask...You say that you currently use .8 gallons of fuel oil per day to heat your domestic water. At $3 per gallon, this is roughly $2.40 per day. You also say that you are using 20 lbs of coal, which even at $200 per ton (assume delivered), this costs you about $2.00 per day. I see what you're saying about equivalent BTU usage, but what exactly are you looking for here?
Your setup sounds very well planned out. I think what you want to do with using the thermocouple to control the fan instead of the grates will work. From a technical standpoint, its as simple as switching a wire, giving the Fuji controller control over a different motor. Actually, you might even look into getting a dual input/dual output controller where you could control both the grates and the fan with the same controller. To my knowledge, the controllers we install are only single input or single output limited (not sure which one right now without checking). But then, the problem becomes where do you want to place the thermocouple to trigger the fan. Currently, we place the thermocouple in the bottom of the coal pile (right on top of the grate) so that as the coal pile burns, the thermocouple is sensing the ash temperature at the bottom of the pile. This is a great way to sense when to shake the grates, but is it going to be sufficient to tell us when to give the fire more air? We use a type K thermocouple for ash monitoring, so it isn't really suited for measuring temperatures much further up in the coal pile. You could upgrade to a type N (a good bit more expensive), but then we run into the problem of where to mount it on our machine. We mount the ash monitoring thermocouple in the grate base where we don't have to worry about going through the water wall. To mount it higher, we would have to find some way of going through the water wall (something that we would have to do during manufacturing as opposed to something you can do yourself). I'll look into it further, but I can't think of a way around this right now.
That leaves us with the question of whether we can get an idea of how the coal fire is doing just from sensing the ash temperature. The best answer I can give is that it might work if you take enough time to experiment and tweak the temperature settings just right. It's worth a shot though... Let us know how it turns out if you do try it.
Also, this is a minor tidbit of info... On the aquastats, if you take the cover off, you'll notice a little tab of metal that prevents the aquastat from going below a certain temperature. You can bend that tab back and go lower if you want. We do it all the time to test the aquastat operation at room temperature. (I don't know what this does as far as the UL listing of the aquastat goes, so if anybody is thinking of doing this in a commercial application or is worried about their insurance, realize that this tip is unofficial. AHS does not condone or support any action not in accordance with the guidelines established in our operator manual.).