My lingering question is how do you service a leaky pipe surrounded by dirt that’s 600C? Do you just have to forfeit a good chunk of your energy savings for the year?

Or is there like a practical maintenance window each year at the end of the winter when you’d do this?

That's not a lingering question. It's the question.

This is a steam boiler. Those are well understood. They have well understood problems. Leaky boiler tubes. Crud in the tubes. Cleaning. The problem here is that you can't easily turn the heat source off.

It's possible to build a long-life boiler for a heat source you can't fully turn off. Every nuclear reactor has one. Heavy stainless steel tubes, precision welding, distilled water. Works fine, but not cheap.

This paper is very hand-wavey about the details of getting the energy out. They're all about the side that puts the energy in, which is the easy part.

Then we talk round-trip efficiency.

You probably don't; you just cap it and accept gradual degradation of the system. If needed, you build another system and connect them in series.