In full disclosure I am a market research consultant so I would recommend handling your issue as a research study. Closed online communities are often used by many companies in many industries and they are generally populated with randomly recruited sample, much the way you would recruit for any research study. The difference is that these research respondents sign on for a more ongoing engagement like a beta test period and use the platform and its tools to post spontaneous feedback as well as participate in more structured exercises.
I think this approach will give you the best, most valuable bang for your buck. I'd suggest you look into something like www.digsite.com to see if it might fit your needs as a platform and they may even help you find sample (whether B2B or targeted consumers).
The value people lose by not engaging a real researcher in these endeavors is the structured, quality control of good study design and management. In other words you could get a non-random sample to give feedback but you would not know how much they mirror or don't mirror the true market and largest opportunity space. Or you could partner with a company or two and end up building a tool that is specific to their needs but not a fit or the rest of the marketplace.
Anyway, that is my two cents. Contact me at email@example.com if you need any advice or assistance any aspect of market or brand research.