As a HS coach, I sometimes talk to the third baseman while I'm coaching 3rd. just exchanging pleasantries. comment on the pitcher warming up if he's throwing really hard, say nice dig by the catcher, congratulate him after a nice play...stuff like that. I appreciate good baseball no matter who plays it. It doesnt mean i have to be a fan of them. Im not cheering for them. but i respect good ball when i see it and ill let it be known. I find that this usually throws the kid off, they want to hate you, but your giving them good reason not too.it sort of "un-pumps" them up. They develop mixed emotions and over think next time a ball comes their way. Originally this was not my intention but... ill keep doing it.
in response to the OP. Absolutely terrible idea. do not do it. you can accomplish the same goal in a much better way. I remember a catcher trash talking me in LL because we had 1 ump who stood behind the pitcher instead of the catcher. I have to think any ump with any professional integrity (at least up through high school) would toss a kid pretty much immediately for something like this.
I teach my kids to talk kindly to the ump and hitters. If hitter hits a close foul and drops his bat and runs until its called foul, my C will pick up the bat and hand it to him and say something like "close one." if the ump takes a foul tip off the mask, my C will ask if he is ok and call time to talk to the P for a sec and give the ump a minute to gather himself. as soon as the game ends, my C turns around to give the ump a hand shake and say thank you. your catcher is essentially your captain, quarterback, ambassador, whatever term you want to use. You dont want your team to be represented in that type of negative light, and umps should not allow it.
-CoachZ