I've seen this done before so I tried to do it real quick tonight by modifying MT-06. Dropped Alpha 4 and Zeta 4 and added Beta and Theta wings with 3 ships each. Kept getting Alpha 3 should be part of Alpha or Zeta wing. Went back to retail FRED and the warning I got there was that each ship in Beta and Theta should be in Alpha or Zeta. OK now there is definitely a bug in the error message in FRED2_Open. The question is should the message even be there?
This opens up another issue. If you can do a 12 player TvT then the custom wing name section needs updated to include the names of the other 2 wings.