Break dust is way less of an issue with electric cars anyway, if they have regen-braking. Don't know how that'd be taxed proportionally though.

I'm also living in a country where road tax is priced per CO2 emission rating (used to be by displacement pre-2006), and am sad nobody's selling kits to capture some or all of the CO2 emissions from exhaust.

CO2 is our largest concern at the moment. Pollution from brake dust and tire dust isn't as important even if it's a large amount.