Music interventions focus on positive psychological and/or physiological benefits for the participants and the interventions will be delivered by qualified music therapists. Also, music therapy interventions neither will have musical developments in terms of increasing technical skills as a primary objective nor will they be primarily concerned with a general increase in artistic activities within the musical domain.
Speaking anecdotally, anxiety around performance is something that pretty much every professional musician deals with at one point or another. I would really encourage your research in this area, as it will be useful to a lot of people!
Anxiety is typically addressed with discussions with the principle teacher, although good conductors play a role as well: a good conductor will give a person opportunities to perform in roles of increasing complexity, demand and importance in order to build the performer's ease on stage and comfort in roles of greater depth. (Think of the actor who plays a bit part before moving on to minor roles, then the lead.) One basic rule is that good preparation and solid technique are extremely valuable, so that skills are reliable in the moment.
One of the most helpful things in my own life was realizing that anxiety is focusing on the wrong thing. If you are really focused on the music at every moment, and focused on communicating something meaningful, there is no room for anxiety. In fact, music is such that every emotion the performer feels will be transmitted to the audience -- the audience will feel what you do! So if you are really interested in the music, they will be too, and it is our first responsibility as performers.
In terms of a methodology, you may want to read about the Alexander Technique. More than body relaxation, the best teachers focus on the mind-body connection and how we can consciously choose our mental state and our actions in the moment, and cultivate ever more refined practice. An excellent book on the subject, written by teachers at the Royal Conservatory of Music, is The Alexander Technique for Musicians, by Judith Kleinman and Peter Buckoke. Another book that deals with exactly your topic is A Soprano on Her Head by Eloise Ristad. Also, I might point you in the direction of Stanislavsky's school of acting; those of us in the opera community know it well!
Thank you for your insights on performance anxiety. I agree with you that focusing on the music rather than the self or the outcome of performance is key. I believe that Attentional Control Theory helps to explain this phenomenon quite well. Great recommendations of the books, I will explore these.
I would look into the research of Dianna Kenny, Aaron Williamon, and Graham Welch. All have written extensively on MPA. An especially fine article was published in Research Studies in MUS Ed: A conceptual framework for understanding musical performance anxiety by Ioulia Papageorgi, Susan Hallam
Thank you for your recommendations-each of these articles are highly relevant, and I have cited each of them in my research proposal. I especially like how Kenny has hypothesized that MPA may overlap with SAD, GAD, and the Panic and Depressive disorders. Each of these sources will form the foundation of my research, and my objective is to develop a conceptual model of how MPA is addressed within undergraduate music schools. Thanks again! Craig