Ahh hah - I must have missed it earlier. You are considering using the sun to heat the hydronic system. I assumed (incorrectly) you were using solar electric panels, storing the electric, then using that electric to heat.
From an efficiency perspective, totally agree that using solar heat collectors vs solar electric collectors is more efficient.
Still not sure that hydronic would be a good idea, high risk of leaks over time and lost space.
So I'll spill the beans!
This revival came about because I want my porch/greenhouse heated to avoid freezing(heated beds).
Water because, a leak is easier to clean up than oils would be!
The mass can be dumped for travel, though the original design used phase changing materials to save space!
Because comfort can be increased, my plan is a dry radiant installation, in the form of mats on the floor to heat my feet where I spend my time while indoors. As I write this the thermometer tells me it's 56 degrees, and I am fine with that, and the heater is off, that's what jackets are for.
With the PV I have, I have more comfort than many people I know and I do that without choking on generator fumes.
For the most part I travel for comfort, and find I burn less resources than those that stay put and try to make their environment liveable!!
All in all, I live considering my impact on mother earth. This requires me, to make sure promises of earth friendly, are not smoke and mirrors.
And don't worry about water damage, because everything will be visible for inspection!