Many years ago I worked for a big university which accumulated many foreign coins from who knows where. The big bank downtown where the U banked (and which had a big forex dept.) took the coins and provided cash for them. But, I was told, that in general, coins are impossible to exchange outside the country in which they originated. And I have found this to be generally true. Big U got its cash only 'cause it was a very big customer.

One solution I have used is to, next time you are at an international airport with a little time, go to the check in area for the country issuing the coins you want to get rid of, grab a boarding passenger and ask them to give your coins to a worthy organization when they arrive. As for Mexican coins, just give 'em to someone you know who is going to Mexico.

Meanwhile, IMHO, don't waste your energy on this.

Good rule, spend all coins before leaving country.