But the advisor told me that perhaps taking a related course from another department would be a sufficient prerequisite if I spoke with the professors of the advanced psych courses when it came time to take them. No guarantees, but it sounded hopeful. So she helped me find what might be a transferable course in another department. She was very helpful. (I wanted to ask her why the heck the department doesn't offer this course I need in a bigger room, or more sections, or do something to match student demand, but I refrained. )

While the professor you spoke to had good advice, I wouldn't wait until you want to take the advanced classes before you ask about if the classes in the other department will qualify you for the more advanced classes. I would ask those professors now how the best way to get into their classes is and if those other classes would get you there. If they won't you don't want to wait to hear that until after you have taken the classes. They might have other suggestions on classes you can take also. They may not be able to promise anything but they can give you an idea. From the horses' mouth so to speak.

Googley, can you speak directly with the professor you are interested in working with? Maybe if he knows how interested you are, he will let you in his group. Or maybe he will have a good alternative suggestion about someone to work with whose research interests overlap with his own.