Is a Minoxidil beard really permanent if beard hair turns terminal?

You are right that no studies have been done in this area. I suspect for a young man some will be permanent and some not. No, it’s no guarantee that simple because a hair becomes a terminal hair it stay that way. 

If minoxidl brings on hair while you are waiting for your beard to normally grow, this is a good way to get your facial hair in advance. If the hairs turn to full terminal hairs, it may be permanent.