There is none as far as "approaching the mound" is concerned. If the ball is live, and he strides the pitcher's plate without the ball, it's a balk. If the ball is dead, nothing can happen.

Bob

MLB Official Rules:   8.05

The rule is different for different rules organizations NCAA, NFHS

MLB Official Rules

OBR - can't be on or astride the rubber. 

FED:  can't be within about 5 feet of the rubber

NCAA: can't be on the dirt.

"Did you mean straddle?"

Thank you. It was late, and my vocabulary has been going down hill faster than my health.

Bob

"If time was called prior to the hidden ball trick, none of these rules apply. The ball cannot be put into play if F1 doesn't have the ball. If the runner is tagged, there is no out. There is no balk. It's a big nothing. I like to say it this way, "It is IMPOSSIBLE to successfully execute the hidden ball trick if time has been called."

1. "IMPOSSIBLE to successfully execute the hidden ball trick if time has been called" -   Not impossible if you have untrained umpires as the OP might suggest. Possibility 2% off the top of my head.

2. "The ball cannot be put into play" -

   It has been and probably will be put into play again 5% of the time (WAG).

3. "There is no balk" - I agree but the last time this happened in my neck of the woods the ball was put in play erroneously and after the pitcher took the rubber and the PU realized that he didn't have the ball, he (PU) balked him. No complaints from anyone. I stopped canvassing and explaining the error of their ways to my umpire group after getting numerous "if he tricked me I'll balk him comments."

4. I don't call balks when the ball is dead but I can't argue with those that do. It's a grey area and only comes into play when the rarely seen hidden ball trick is improperly attempted.

Bob:

You ARE slipping. Welcome to the club.

Check your gun at the door.

8.O5(i) reads "astride" not "straddle."

Rich (himself) uses "astride" in a later post below.

But either word basically meaning the same.

So your vocabulary is intact with your use of the word "astride." supported by the wording in 8.05(i)

Frank!