Is CollAction kind of what you had in mind? Wikipedia also mentions PledgeBank which closed down in 2015 but still has a site with all the archived pledges at www.pledgebank.com. Not sure but I don't think either of these sites allow versions of assurance contracts that use financial contributions.
I wonder what a good binding mechanism might be for crowdaction sites in general, because once the quorum is reached you still have to get everybody to go ahead and perform the action. Any thoughts?
A site based on dominant assurance contracts would also be interesting. It might be a bit more like Kickstarter, in that maybe the entrepreneur initiating the DAC can market and popularize the action, and be responsible for the action happening when/if the quorum is reached.