Most insurance will "come around" if you can document at least 3 treatment failures of 3 different drugs or procedures. Treatments are usually considered a failure if they do not produce results within a reasonable amount of time, usually 3-6 months.
If you have not tried 3 and failed those treatments, there will not be a whole lot that can be done. IVIG is very expensive and unless there is a need, they will not approve it.
The "experimental in MG" line is so old. I've been getting that one for over 10 years. Luckily my doctor was in on the original research for IVIG with MG, so she just sends paperwork and all is good again. And, FWIW, I have had Aetna in the past.