I need a fire sprinkler design to get a building permit. (Finally have blueprints!)
The town's ordinance, which requires NPFA Section 13D standards, calls for a 300 gallon tank to carry the house for 10 minutes until the fire dept arrives. Because I have no basement, the tank would go in the garage, along with measures to keep the water at 40 degrees or warmer.

As an alternative, I could use my planned reverse indirect tank for two purposes. (Reverse means that the water in it never changes, is maintained by an expansion tank and makeup valve, and used to preheat DHW and hydronic through heat exchangers.) The heat sources are solar collectors and a heat pump water heater.
300 gallons is probably too large, but any contribution would make a second tank for sprinkler purposes that much easier to fit in.

Any arguments against this from an HVAC point of view?  Actually the DHW and hydronic would still work with an empty tank, with a tankless electric doing all the work.

Does anybody even MAKE a 300 gallon reverse-indirect? There are some 120 gallon versions out there, but that's about it (and they're not cheap.) Or are you talking about a ~50gallon reverse-indirect (also not cheap) coupled to a 250 gallon sprinkler tank?

You'll have run the reverse indirect at well above 40 F any significant pre-heat benefit out of it. I love Rube Goldberg contraptions though- can you sketch it out in a bit more detail?

You know my Uncle Rube?
Over in the outdoor wood boiler world there is much interest in reverse indirect tanks. These folks make them into the thousands of gallons: www.stsscoinc.com. Of course, there is also www.kotly.com, where they're known as accumulation tanks.
Here is where I am now on heating after considerable evolution: Passive solar first, recognizing that the sun doesn't shine much in Pa in Dec. Wood stove boiler second, plumbed to radiant pipes in a concrete slab, and capable of adding 10 degrees to the slab in 8-10 hours of efficient burning -- strictly seat of the pants regulation.
Third, thermostat controlled radiant in the bedrooms, on the second floor only, operating on water storage. The water will be heated by solar collectors and a heat pump water heater. The HPWH should operate at high COPs. The wood stove requires a heat dump radiator, which will be adjacent to the HPWH. Ditto for the stove pipe. Passive solar should give it a boost as well. The HPWH is located on the second floor of a "heat tower" plenum that moves air between the floors.
My goal is completely civilized passive solar and wood heat, in which all occupants remain fully clothed at all times. AAC walls and concrete slab add up to 50 tons of thermal mass, which will further buffer periods of intense heat.
Dunno if the HPWH will stand in for an air conditioner in the summer. (I want predictable temperatures in the bedrooms and bathrooms.) Also dunno how well this works before the leaves fall in mid Nov, until which point passive solar won't work.
Needless to say, I'd hate to see 300 gallons of water just sitting there. I'd treat it as auxiliary storage in a two tank setup, setting 110 degrees as a transfer point, say, so that smaller tank heats DHW reliably. I'd need a heat dump to keep it at 140 degrees or cooler. Wouldn't want the sprinkler heads to go off.

You can do some time shifting with water tank heat storage, but 1000s of gallons would be more useful.

Turns out this is a very bad idea from a safety standpoint. In an accidental discharge, or a flow between you and the exit, 140-degree water isn't much of an improvement over flames. For what's it worth, the HVAC side is workable with swing check valves to stop thermosiphon in the sprinkler lines and a circuit connecting heat exchangers in each tank that allow you to treat two tanks as one (the last courtesy of STSS.)
Oh well.