Hum...
I couldn't successfully add a trance to Beatrix ; you need at least two things for that:
1) Making this character a permanent party member, that is removing the flag "temporary character" in the "SetCharacterData" call (in fields scripts),
2) Add a trance model to the list of character models, in the CIL "btl_init::..ctor" method.
With that, however, the trance gauge appears and fills correctly, but the game freezes once it's fully filled. So there has to be something else that I didn't spot :/
Also, the "temporary character" flag determines whether the game will use the temporary character or the permanent counterpart (Marcus/Eiko for instance). So for adding trance to Marcus, Blank or Cina, it would also require something else (maybe changing the condition (1) for the trance bar to appear).