One of LFortran authors here. Yes, we already have an open issue for exactly this: https://gitlab.com/lfortran/lfortran/issues/42. As you can read there, it's actually not that easy as I first thought. But I think it's very much worth pursuing.

For example, Python's AST is not round-trippable, and that forces people to write alternative parsers such as Parso (https://github.com/davidhalter/parso).

ANTLR should allow to parse all the white space. But the issue is how to represent it in the AST, see the issue for more details. One could have an AST and a parser just for this round-trippable application, but that defeats the purpose. The goal should be to have this part of the compiler so that one can trust that it parses things correctly.

Right now I am concentrating my efforts to finish gfortran compatibility (see the roadmap at https://lfortran.org) and to get first users. Once we get further along, we will tackle this problem. I am hopeful there might be a way to do this.