I can kinda see the merit here, although there are a couple of things that could be a tough sell. Specifically, creating an unstable branch means that we would have to prepare interim "releases" regularly; we couldn't just give access to the dev branch because there's often stuff in there that would potentially break the game for anyone that wanted to legitimately play it.
Taking the time to do that, and make sure it's playable (key word there) enough so that you could give us real, usable feedback is bandwidth and time that we really don't have, speaking from my perspective. There's also the time necessary to parse all of that feedback on a regular basis into reports, to figure out what's valid and/or relevant, what's not, etc. This is why QA departments exist -- it's a controlled environment where reports are streamlined and can be condensed. You wouldn't want to know what our dev tracker would look like if we allowed the entire community to file their own reports
In terms of having an unstable branch, how often would you expect builds? Just curious.