There was a
similar thread last year to get all armor to act as light armor, but I don't believe we've ever tried to remove proficiency from armor...
What you might want to do is make the "magical armor" actually a wondrous item that changes your unarmored AC calculations. Have a script on it that checks to see if you have armor on and throw an error if you accidentally equip some and not run the armor bonuses until you take off the offensive armor.
You can look at the monk or barbarian unarmored defense scripts to get an idea of how to check for armor and manipulate unarmored defense. They'll take a bit of manipulation to put on a magic item, but that's the best pointer I can give you.
*Hint, go to the class special tab in the editor and do a new(copy) on unarmored defense and you have a free copy you can dig into and reverse engineer*