Comparatively recently, they let mechanics and quick change oil places do the test for 2005 and later cars, giving them the option of charging a fee for it.
Oregon doesn't have vehicle safety inspections but it doesn't seem unreasonable that I could go to the official DEQ or DMV locations if I wanted to have my mileage inspected or maybe pay Jiffy Lube a bit extra because they're closer and their hours are more convenient.
The issue with that is that, though, is the same problem with taxes: many (most?) people aren't going to want to come up with the money to pay their mileage bill on the spot. There don't seem to be great answers for that.
Oregon has a pilot program where they put a device in your car and charge your credit card 1.9 cents per mile. https://www.myorego.org I'm sure that's what they'd like to have people do but I don't know how many people want to do that.
Maybe some yearly assertion of mileage and random selection rate yearly would suffice.
Not that I'm advocating a Orwellian tracking system be installed. Just saying I can see how existing systems could be argued against.