I had the same issue and filed the 843, which was rejected by the IRS as the inappropriate method to claim the excess SS.  BTW, I filed an amended return before I was aware of the 843 and that didn't work, obviously.

We appealed to TAS to resolve this issue.  The TAS rep agreed with us and tried to get the 843 processed.  But the examiner just rejected the appeal, claiming that a 1040-X IS the correct method, and we should designate the employer with a CPEO (Certified Professional Employment Organization) exception.  I can't find any reference to this exception in ProSeries.  

Any thoughts??