Every school will allow you to do electives wherever you want. It is hard to find anyone who will take a 3rd-year student for an elective if they aren't part of your affiliate list, but for 4th-year, you can do whatever you want.
If you look at the websites for various programs, they usually say that they only take 4th-year students. Sometimes you can get them to take you if you've got an elective really late in 3rd-year, and you've got all your core rotations out of the way.
What I have told you is true for every osteopathic and allopathic school. Core rotations and 3rd-year will be done with your normal affiliates. 4th-year electives wherever you want.