May I suggest to break out smaller bounties. It's $290k to fulfill a seemingly herculean task that requires to assemble a team and work out many details that you have not defined and neither does anybody know what others are working on. How about awarding $10k of that money for the 3 best proposals (5k, 3k, 2k) to tackle the big task in time and budget. These proposals have to detail sub-tasks that again can be accomplished in short sprints. Pay out +$10k every week or two so people see it's real and in reach.