A 51 modifier is for surgical/diagnostic procedures, not for therapy codes.
While I don't bill for PT exclusively, I do bill some PT codes for my doctors. To my knowledge, almost every CPT code that relates to PT is a timed code, applied to one or more areas. So you can have a PT doing a 97110 for foot, back, and arm, and what matters is the fact that it is billed in 15 minute increments, not how many places the therapy was done on the body.
So I guess I am not sure I understand your question? Can you explain a bit more?