I don't think that it's that Square won't let Taito out to play, I think it's that Taito's library makes it very unlikely we'd get anything from them above the level of supporting content. Space Invaders is obviously storied enough for a character, but by nature of the game and its characters within, that seems a pretty unlikely proposition. Bubble Bobble is (...was) noteworthy, but not really character-level. They'd make a great AT, but Square is indeed kinda restrictive about content, so that also seems a big big if.
But in theory, if Space Invaders was more conducive to playability, they'd have a real good shot. It's not like Square bought Taito to kill their franchises, they would benefit from the inclusion too.
As it is now though, I wouldn't consider anyone from Taito even in the like... top five most likely just from Square alone.