Possible Model (Score 1)

Here's a possible model that came out of a little brainstorming.

-A User creates a wish list item for a given Project.
-The Project Maintainer, whether an organization, company, or individual, controls the cost estimation for the new project feature. No other developers may submit estimates (ie bids). These estimates include a dollar amount and a time to completion.
-Users pool their funds to pay for the given feature. These funds should be held in escrow to ensure they actually exist when the estimation amount has been reached.
-The work is completed

This approach varies from the other approaches primarily by having the Project Maintainer act as the point of control for all development. This allows the Project Maintainer to allocate funds received for work done between developers and other overhead costs incurred in the general maintenance and development of that app in general. This also ensures that the code will be included in the primary distribution and supported in future releases, meets the development standards, and rewards those associated with the project.

I have left out accountability on the part of the developer...I imagine there could be some contention between those who made and paid for a feature request and the person implementing it...One group saying the feature is not done as requested and the other claiming conformance to the request...but that's just a detail :-)

I'm not sure it extends well to custom fittings for a specific user need, but I think that sort of work can be addressed as well with a more traditional bidding model. There is much less need in the customization case for the code to be included in the primary distribution. I do think the Project Maintainer should still be considered the preferred developer for such work.