I had nearly this exact idea before. Actually that's where my username Avalaunch comes from - "launch your new idea with an avalanche of support". Thus far I haven't followed through with the idea because I don't think it provides enough value to the would be early adopters.
Two suggestions:
1. Let the idea author dictate how many users are required instead of setting an arbitrary number. For some ideas (new dating site), 500 isn't nearly enough. For others 50 would be plenty.
2. Set it up so that the idea author is encouraged to give something to the potential users for upboating an idea. Ie. "First 3 months of service are free for all upboaters".
Good luck! I'd be interested to hear how it turns out for you.
EDIT:
Suggestion 3. Add login by Facebook. I know most of HN hates it, but I hate filling out forms more.
EDIT 2:
The develop upboat is a really cool feature that I hadn't thought of. Maybe add a designer upboat too? Or ideally, let the idea author determine what kind of upboats he's looking for. Maybe I have a great developer but I really need a salesperson.