Running water down hessian walls is good. I had a mate years ago on an outback property, they had a cool room with hessian walls soaked by a tank on top and some drip mechanism that I can't remember now. It worked pretty well but was bore water so stunk like rotten eggs, after a while you don't notice though.
Another thing to do (as you mentioned) is use the "deep earth" temp that never really varies and rig up a heat exchanger. Just bury X metres of pipe in the ground and circulate between that and a "radiator" in the house. Because it's a closed system there's very little energy used to move the water and you don't have the legionnaires problem.
In Aus we have towns where half of the people live underground like something from star wars. You buy your land and dig into the side of a hill to make a house.

This is a shot of the hill above a motel, the pipes are ventilation for the guest rooms.
On a similar note, check out this hot water system

A role of poly pipe layed out in the sun. It works a treat and I actually recommended this to an Aid agency working in Africa a few years ago as a cheap low-tech method of getting hot water. Apparently it did the job.
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Rob