Cars already have sensors that can tell if you're sitting in your seat or not. So avoiding this is just a matter of putting the tube you blow into in such a position that it's almost impossible for someone not sitting in the driver's seat to blow into it; also you could require that the passenger seat is empty and both front doors are closed.

Remember, the breathalyzer is only a backup in the situation where the car has detected alcohol vapors in the air, and is wanting to confirm that the driver is not the one who has been drinking. So it's not something you do every day, ergo it can be a bit of a hassle.

Hobos aren't generally someone you'd want to rely on for their sobriety.

They wait for a couple of ten buck payoffs before retiring for the day. Or the union has a designated blower for the day.

I wonder how unique the chemical makeup of an individual's breath is