I don't want to be cynical, but a petition will probably do little although it is a good starting point. We're trying something similar in Henley (see my signature) and we've only just really started to progress and that's after turning up to a few council meetings, holding our own first public meeting and having our own project group meetings.
Have you faced any opposition already? It should be easier for you considering the council have already agreed to invest on the BMX park presumably, but still, anyone can go out and get a few hundred people to sign a petition. Make sure you make your case well, showing how it benefits the community not just for you riders.
I'll sign the petition later by the way, cool to see Danny Mac signed it (if it is him )