There aren't really any triggers set up in legacy pay as you go for reminders and the system for pay as you go isn't sophisticated enough to pull a payment when you dip below a certain amount, unfortunately. You should be able to set up an auto refill through my.t-mobile.com with these steps Manage automatic payments, however it will be on a monthly reoccurring basis, not based within other parameters.
I hope that helps, even if not exactly what you were looking for.
Thanks. I guess we stick with a note on the kitchen calendar and periodic checks of minutes remaining.