I don’t know about music therapy specifically, but I know a number of people whose plans involve art therapy (specifically dance and visual art), and I think their classes mostly involve a mix of art and psychology.
The Plan is about developing a line of inquiry and then pursuing that through different disciplines and from different angles. So I imagine the majority of your classes would be music and psych classes, but then I could also see you taking some classes about say the role of the arts in public action or about music education philosophy. It’s all about where your interests lie, and the sky’s the limit as long as you can make the connections.
Hope that helps,