It depends on how serious you want the league to be
If the finals were determined on only one game, then I would not allow Hercules to be picked as it could be disputed whether it is "real pinball". but if there were more than one game in the finals, and an odd number of games such that it could one of the tiebreakers, I'd allow it.
Quoted from batcitypinball:
Hercules actually was used in a finals match in a tournament in Hockley at Dan's pinball museum. Brian D and Sven battled it out. They actually both killed it and it was a highly competitive and fun match. However, I think that Hercules was a bit faster and more "pinbally" than the one at Pinballz. Brian might chime in.
Hey that's me. It was interesting. One of the players earlier in the tournament couldn't help but show off how you could save the cue ball when it went down the left outlane by repeatedly flipping the left flipper as if you were dribbling a basketball and you could bounce the ball off of the bottom rail onto enough of the right flipper to save the ball.
Did I do that on ball 1 in the finals? Yes, I did. And then I heard people in the background go "was that a death save?" So that was funny. On ball 5, Sven did the same thing and won the match.
If you buy me a few games on Hercules, perhaps I can demonstrate the move for you.