There are a few examples of this idea, like DARPA's AVM / iFAB http://arstechnica.com/business/news/2011/12/darpas-factory-of-the-future-looks-like-open-source-development.ars and Local Motors http://www.local-motors.com/
Interestingly I'm having a hard time finding specifications on how much of the project is open vs. closed source and how much of the decision making is done by professionals as opposed to collaborators. It sounds like DARPA at least is basing the design decisions on a pre-defined entirely objective equation. That solution, however, presupposes software powerful enough to analyze the design based on an equation. I doubt they're going to make their META tools open source. Maybe they'll make their cyber/electro/mechanical description language open source.
I agree with the coherent, simple, free criteria. Although i think we'd need to be willing to compromise somewhat on the "simple" part. It seems like there should be reasonably obvious boundaries between the components of the toolchain. For example, only a few people are going to be at all competent at modeling the designs in CAD, so it makes sense to not require that program to be easy for anyone to understand/use. The same applies to the project management activities. On the other hand, gathering reference documents, and distributing reference documents, and building finished documents, and performing the actual tasks, should all be low-barrier-to-entry.
For example:
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