Determining demand for unfinished products

Once a product has moved from the idea stage to the prototype stage, Kickstarter and similar sites are great ways to determine demand. But what about products that are still at the idea stage (but which are obviously feasible)?

The domain I was thinking about recently was audiobooks. I bet there are lots of potential long-tail audiobooks (especially older books) that aren't getting made because of uncertainty about demand. The existing systems to determine demand using the internet (that I'm aware of) likely don't allow many audiobooks to be published.

• When an author tries to determine demand they will sometimes use a crowdfunding site (e.g. indiegogo which is very permissible in the type of project funded) to gauge demand. But this is very rare and I think authors of older books are very unlikely to do this. 
• Amazon.com allows people to note their interest in an audiobook version on the book page. But amazon doesn't know how serious you are since it's practically costless to click that link. They also only use the data for make audiobooks through Audible.com which has a high production cost (potentially voice actors on ACX might do it for less).

What would be great was a site where people could monetarily show their interest (similar to indiegogo) on any potential audiobook and the site would track progress toward the goal. It would be more like bounty programs since the end-goal isn't specified and the producer hasn't been determined yet. Then entreprenuers could contact authors about audio rights (or authors could do it themselves). The site could even have free copyright as the end-goal like unglue.it for audiobooks. If that were the case, voice actors from LibriVox might record the audio for free and the payment would just be to secure the audio rights.