> In theory, one should be able to use OpenSCAD to come up with fancy surfaces to 3-D print, right?

Yes, but it is painfully slow. Even perforated patterns are quite slow to generate.

OpenSCAD nightly using the Manifold engine is a lot faster than the CGAL crap the stable version ships with

Aside from Fusion360, is there a Free (or FOSS) cad package that uses breps and is scriptable?

Fusion360 is just stupid fast at perforations and sophisticated modeling constructions via its python API. I use it because it works well, but I'd be happier if I didn't have to maintain that Autodesk dependency...