I too have knocked a mirror off of a vehicle however my case ran a touch different.
I was on a workout (not commute) ride and was cranking up a hill when 2 rednecks honked and yelled... you know it. I cought them at the light and confronted them verbally. After explaining how we had equal right to the road and so on he threatened me, twice. Said he would run me over, so I told him he would go to jail.
Then he tried, when there was about 6 inches from his truck to the curb I decided I was gonna hit the whole truck or knock off his mirror. I hit the mirror and he sped off leaving it behind.

Although I don't think your actions were the best thought through, perhaps there were alternatives.
Nor do I think my actions were the best thought through, there was a ton of adrenalin. (especially because I pulled and cought him at the top of a mile long hill in a 35 mph zone)

Assuming they were high school or college age (as I am) I know this would be talked about with their buddies, and everyone has at least one buddy who would try to explain how wrong the driver was. Who knows if theywould ge through.

Just be careful there, nobody wants to see a cyclist INTENTIONALLY run down... It's sad enough when it's an accident.