They don't need to mandate it, at least not at first. They can just incentivize it the right way and people will willingly jump on board.

"Install this app on your phone to make traffic lights recognize you when you approach them, for a timely shift to green light"

"Install this app to collect pedestrian-points, exchange points for prices and discounts!"

"Get a better insurance tier for installing this app which tracks your fitness. Do your daily 10.000 steps for a free <whatever>!"

Once enough people jump on board, and it's generally accepted as being "normal because everybody uses it", then you can mandate it and the few people left who resist can easily be branded as "paranoid tinfoil-hat people" to marginalize their opposition.

If you're relying on a smartphone to host a beacon, isn't that already a solved problem?

Smartphones are radio transceivers, I would imagine any anonymous beacon technology wouldn't be noticeably louder than what smartphones are already emitting.