I wouldn't mess with the tires. It creates financial incentives to compromise on safety by using old tires, avoiding buying separate summer/winter tires, may create a black market for untaxed tires without quality control, etc.
Then you can do what was done when these things came up for emissions and have tire checks along with your smog checking. Tread depth doesn't even need a fancy inspection site. A parking officer could easily be checking tread depth while they are also looking for double parked cars and out of registration tags.
