I have no useful experise regarding off-label prescriptions.
I think it will be possible (once approved and available for Rett Syndrome).
Two complications, perhaps.
First, many physicians will simply stay away from early off-label prescriptions because of medical tort laws and medical insurance. Untoward outcomes opens the doc up to zillions of dollars of liability.
Secondly, off-label prescriptions not covered by insurance; so expenses for such will may be sky-high.