My brother in a distant State had a similar problem and his contacts had no solution to offer. So he asked me. He had a point-of-use heater in the bathroom and that was the only hot supply for his 75 gallon tub. The tub was not used often. The point-of-use heater had such a low flow rate that the first water into the tub had cooled by the time the tub was filled and the entire thing was chilly. He also had only a low crawl space under the house for adding any equipment.
So I did all the calculations on the basis basis of the BTU required-- ergo, the tub required X BTU for a fill of Y gallons at temperature W. His plumber determined the largest electrical water heater he could find that would fit into the crawl space. I calculated the needed temperature for that size heater to store the required X BTU. And it turned out that it would have X BTUs at temperatures just under the maximum possible setting. So the heater was installed, becoming the only hot water supply to that tub. Power to the heater is controlled by a relay which is controlled by a wall switch in the bathroom for the occasional use. Keeping the heater hot all the time is possible if desired. He turns on the heater about 1 1/2 hours before using the tub. The heater supplies the required heat of X BTU and the cold water supply to the tub makes up the remainder of the required water volume.
That's the procedure. You can do your own calculations.

Install 2 water heaters in series, with total gallons equal to demand requirement.
http://waterheatertimer.org/images/Two-water-heaters2a-300.jpg
First water heater receives incoming cold water from outside.
Hot water outlet on first water heater is connected to cold water inlet on second heater.
Hot water outlet on second heater supplies household hot water.
Use 240Volt timer on fist water heater to schedule hot water usage for large bathtub.
When first heater is turned off, it acts as tempering tank in winter to passively preheat incoming cold water.

Here is a link that might be useful: Two water heaters