Three points are key and Mitch mentioned two of them in one sentence:
"It's easy to over estimate demand and under estimate cost."
The third point is this: whatever is made, will people like it?
If some headphone is made, how much "planning" would need to take place? What level of participation would the "investors" want to have in conjunction with the mastery of the manufacturer? John knows how to build headphones, some love the sound, many like it. If someone were to invest in a Grado special edition, with perhaps a handful of requests (screws, all metal, supple leather, etc) but leaving the overall sound to Grado Labs to figure out, would that be enough?
Certainly a few things would need to be worked out:
1) a manufacturer willing to take this on (maybe Grado Labs actually would be, they are by far the closest to the Head-fi community on a person to person basis)
2) a clear understanding of costs. Would a deposit be sufficient with the final price to be determined once the product was finished? Would there be heavy communication back and forth during the R&D and then manufacturing phase? With the PS-1, much of the parts were "recycled" from the Free systems headphones. Developing something brand new might actually cost quite a bit because of machining and tooling. (if folks really wanted metal).
3) hard or soft deadlines?
4) who owns the rights to the product?
5) limited run or ongoing product line?
Personally, if I could be a part of something great I wouldn't care if the manufacturer "owned" the rights, so long as minimal profit from the initial investors was extracted from our pockets. If the manufacturer is acting as a contractor, then the Head-fi members should own the rights to the headphone and get some type of profit unless agreed otherwise. Basically, I don't want to invest a ton of cash only to have the manufacturer make off like a bandit from my initial investment AND then make a good deal of money off of future sales. At the same time, if I was assured a quality product for say $2000 invested, for me personally, I couldn't care less. I wouldn't be in it for the money, but some might be and/or would think of this later on if the product was really successful.
At the end of the day the initial three points are the key:
How much will it cost to make, will there be enough folks up front to cover the costs and once made will they like it enough to be satisfied? Would there be an alpha and beta testing group to ensure the product hits the desires of the majority?
Lots to think about before anyone should be approached but certainly I would be willing to take the lead in organizing something "official" with whichever manufacturer was willing to work with us.