sweet and stable

The goal is to create a permanently sweet and stable cider without adding anything to the cider. This means halting fermentation. Cold-crashing works but is not permanent.

One idea I had was to stop fermentation by killing the yeast by heating the cider up to 140 by running the cider from one carboy to another through copper tubing that would heat up the cider and then cool it down before depositing it (how's that for a sentence?). This way you would kill the yeast without losing aromatics. Despite being very complicated, and probably costing a lot to set up, I'm guessing you would still be messing with the pectin and wind up with a cider that won't clear. I'd love to try it out, but I'm not sure I want to dump the money into the set up.

Anyone ever tried something like this? Any other ideas?

I know you don't want to use copper with finished beer. I'd stay away from copper with finished cider, as well.

Why not bottle the cider when it's where you like it, and pasteurize in the bottle following the sticky in the top of this forum? That would work, wouldn't require equipment, siphoning, pumping, etc. and it would work.