You want to basically be able to add in stuff to block a path... Well...
I know of ONE such thing: Geysirs with/without refineries/extractors/assimilators.
Someone here once made a 64x64 map that had a bridge that was blocked by a geysir, but if you built an extractor on it, zerglings were able to pass through. Dunno who it was... Maybe you yourself, Nightmarjoo?
See, as long as there is an extractor on it, the geysir takes up less space. If you destroy it, you tighten the path. So two extractors close to each other could serve as a path of variable width. Would need some precise testing, though.
Oh and there also are those crystals damn-I-can't-remeber-who-made-that-map wanted to use as scv-drop-bombs. You can shut off terrain with those as well.
Another idea: something ALMOST unvulnerable would be some of your units below a swarm/DWeb combo. It might be hard to place them so that it works, though. modified by spinesheath