I'm happy to report that this has now been agreed to, and to brag that, due to my awesome diplomatic prowess, the negotiations were speedily concluded in only five months.
Here is the system schematic. It is a drainback design that will provide both domestic hot water, and space heat via the in-floor hydronics. The parts that currently exist are the electric water heater, boiler, and radiant floor loops on the left, and the solar panels on the upper right.
This is essentially a tailored version of the design recommended by the solar collector manufacturer (Solar Skies.) There is an extra 120 gallon heat storage tank because of the large solar array (ten 4x8 foot panels.) In this design the large storage tanks could function as the domestic hot water supply, but because there is already a small electric hot water heater, the main tanks will serve as a preheat for it.
Here are some ballpark calculations I did on the heat demand, supply, storage, and distribution.