Actually, a fixed gear has been ruled to be an acceptable brake in past cases, due to the fact that you can lock it up. The HTA is not terribly specific, fortunately.

When I lived in Vancouver, about 10 years ago, a co-worker had a ticket dismissed out there for that same thing. She commuted on a track bike (she actually raced at the track a lot) and won a ruling regarding a fixed hub being a legal braking method.

Don't count on Metro's finest caring about the law, though. If they get told to write tickets, they do. Just take it to court afterwards.