Thank you so much for your email, and I hope I can shed some light on the situation for you. Dancers and especially high level ones are often a tricky bunch to treat and can push you outside the bounds of treatments for regular clients.
I am not sure what facilities you have access to, but if you do have access to a visual ultrasound that you can use to visualise her lumbar multifidus, then this would be a great start. Especially if she has had pain for some time, and has a non-union in the TP… Is there any chance that this may be an old fracture that has failed to heal? Or is it a congenital or developmental defect?
If she has had pain for some time there is a high likelihood that the local stabilizers at that level will be inhibited. This often occurs and can be overlooked in rehab. This would fit with your reports that she gets pain after dancing for long periods. I am not sure if her pain is an isolated joint level pain, or a more diffuse, general pain in the global musculature of the region. If local stabilizers are inhibited this leads to overuse of global stabilizers to compensate, and often results in pain.
We have a specific program based on isolating micro movements at each level in the lumbar spine to facilitate local stabilizers, that is a little difficult to describe in an email. A basic version of this is in our A New Approach to Core Stability.
I would start with an assessment of the muscle bulk via palpation through the lumbar musculature, with visual ultrasound review if possible. Initial rehab should be focused on conscious activation of multifidus in neutral, and then the introduction of Rotatores and Intertransversarii as rehab progresses.
It is essential that the dancer learn to facilitate the use of these muscles in her basic (and then more advanced) ballet technique. This is especially necessary when she has had pain for some time, as adverse motor patterns can be set up that can take some reprogramming. You have to be very careful with dancers, as often they are very good at masking weaknesses by bracing with global stabilizers!
Please let me know if I can help out in any other way,