Right, even the most opinionated of modern formatters don't go quite that far. The only thing I can think of that did is the Interlisp-D editing functionality

Right, even the most opinionated of modern formatters don't go quite that far. The only thing I can think of that did is the Interlisp-D editing functionality provided by Interlisp Medley, where your code was stored within the system as parsed S-expr trees and when you opened an editor onto something it reverse engineered a textual expression of the thing according to your configuration. Tagging every, single, node, with enough data to recreate the indentation and formatting seems theoretically doable, and if you're wanting to let people edit things that are, say, config file shaped, then you're going to have to do that if you want the resulting commit diffs to be sufficiently coherent that people will be willing to use it. But taking the Medley approach for a first attempt/first release feels a lot more doable, and given the biggest obstacle to such a thing is almost certainly congruent to "being able to make a start over a weekend for the sheer fun of it" I think it's well worth considering :D