Root
When a building has a form of the "Root" ability its facing angle can be changed. Just add a version of the "Root" ability to every building you want to be able to rotate and disable the ability it on map initialization.
If you can't get it to work, I'll create you a template map.
PS: Changing the "Is a Building" function causes massive lag, at least for me, and if I don't want a unit to be a building I don't base it off of one.