For what it's worth I've tried that (here in WA state, in the US) and most colleges (including mine) don't have a test proctoring center. Occasionally you'll find one but the center will frequently will only proctor tests for their own college (i.e., it's not public service - it's just for that college).

So here in WA, USA there's definitely a system for this. If you don't mind sharing - where do you live? Have you used this proctoring-for-other-colleges system? How do you know about it?