Based on Maaser Sheni 3:4, you can redeem maaser sheni coins on produce in Yerushalayim even without bringing the coins to Yerushalayim.
I have in mind a system where you deposit coins and they add money to your account. Then you say "when I buy food through this system, these coins should be redeemed on it" (this kind of formula works even when the food doesn't exist yet) and they put your money in a labeled compartment. You spend the money from your account (using an app or membership card or whatever), which only works in Yerushalayim, for food in stores owned by chaveirim or for live animals. When you use up the money, they take the original coins out and keep them.
This would be a lot easier than carrying around lots of cash and paying stores with it. And it would help in tracking how much maaser sheni you've spent. Is there any halachic reason it wouldn't work when Mashiach comes?