Ya know. I just realized yesterday that there is no Script value for limit gauge. You might be able to force him to use a limit using
60 14
60 XX
92
73
I'm not sure what XX would be. Either 2D or AD. Most likely 2D. That would force him to become Galian Beast. You could also add a random amount in case you want it to be different every battle. I'm not sure how this would behave when encountering enemies (bosses) that have pre-battle scripts of their own.
I think he'd also revert to Vincent if he dies.
Anyway, it's all possible to do.