> Do you know of any free/open-source examples that solve this problem well?

No. Sorry, I should have been clearer. None of the open source programs handle this.

> These heuristics seem like exactly the sort of thing that commercial CAD applications can afford to spend resources on, and that open-source community-driven applications would struggle with.

This seems like one of those things where "the industry" converged to a solution and the people outside of it simply don't know what it is.