Very nice dude, I was a trade mason, and I also skate. I helped build a few DIY spots here in NY.
The lack of power would make hydronic harder but a small circulation pump wouldn’t draw much juice, sub 50 watts, you would be able to power that off of very modest solar. You would still be utilizing the concrete to transfer the heat just letting the pipping inside of it direct the heated water, instead of air ducts.
My main worry with the exhaust gas system is if the concrete ever cracks from the thermal expansion, carbon monoxide exhaust gas could flood the greenhouse and kill everything.
I get the keep it simple approach though, just wanted to throw extra ideas out there for you to think about.
I have always wanted to install a super simple greenhouse structure over my local DIY spot to keep it warm and dry in the winter.
Keep ripping 