The managing site can just hold them until the project is satisfactorily completed. If you wanted to eliminate even the risk of the site stealing from you there could be a confirmed and unconfirmed bounty.
Seems like the ideal task for Gavin Andersen's ClearCoin
. If ClearCoin collects a percentage of the escrow for acting as a mediary (don't see it mentioned anywhere either way), perhaps it could waive the collection since the escrow is going to develop the official bitcoin project.
After taking a quick glance at the site so far, I strongly suggest that it be tied directly to the official bitcoin new features and bugs
github page. Users submit feature requests and bugs through the official channels, improving visibility for the developers, and the project-support list more closely follows the active, regular, development. If it is not tied to the official list, then unnecessary effort is imposed on developers since they have to monitor multiple lists. Having the bounty list based on the official "issues" page also improves rewards going to the right people.
I suspect this was your goal all along, and the existing site is more a proof of concept.