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.