Heat pumps are a nonstarter here, because the goal here is minimizing capex, not maximizing efficiency.

Sorry, that comparison to heat pumps wasn't meant to imply that heat pumps would be used in this system. Rather, the focus was on the pipes that transfer heat from the system to the ground and back. That component remains basically the same — the heat pump components of a GSHP are above ground. My thought was that experience installing GSHP would allow you to bracket out the cost of the interred heat exchanger and make a comparison to the title proposal.