First, on the fraction of a sec early - even in a 2 umpire game, when I am on the bases, I need to actually
see the runner leave early, and I don't have stop-action instant replay available. Most agressive runners will start their motion in anticipation of the pitch. I will watch for ball-leaving-the-hand, foot-losing-touch-with-the-base timing, but it has to be a clear violation before I'll call it.
I have had travel coaches try this, and I have adapted my stance to watch for it, but what got my goat this time was that is was 12-bleeping-U fall ball with no standings being kept! Besides, the team wasn't that good - there was a lot more useful things that coach should have been working on.
We have this funny anomaly here with fall ball; during the season itself, no standings, etc. are kept, games have a drop dead clock, so games don't matter except for individual team pride, but it does end with both a state qualifier tournament and a state tournament. I'm calling the 16 U state qualifier this weekend. These games do matter, and these players are generally quite good. And, they will be 1 umpire games (as I said - anomaly). I do expect to be challenged with aggressive play.
On WMB's plate meeting warning - it seems to work for him, and if said in a half-joking manner, I can't really see any down side (unless, of course, you actually follow through with the threat).
I kinda like the asking the coach to call it idea. I especially like the idea of asking a coach who is sending his players early to call his own players out for leaving early. I have no idea what he would do with that, but just having him struggle with the ethical dilemma would be worth it!
It might be something to ask him in front of his bench so his own players, and maybe parents, would hear the request!