Phone aren't always on, aren't always in coverage and the app would have to be actually active when the phone is switched on anyway. How do you think it would work?

It would be a reminder app, and otherwise work exactly the same way as those creepy "spouse tracking apps" people use to catch cheating housewives.

Unless you plan on surgically implanting the phone, I don't think that's a bulletproof plan

I dunno have you seen human beings lately? Most people seem to be surgically attached to their nerd dild^H^H^H phones. Assuming the bailee is a rational and well intentioned actor (the article does seem to imply many people miss court because they forget), this is probably a reasonable thing to do.