The big question is, would Spyderco be willing to do it that way. I don't think they would. The idea has come up before (group buy) and they said no for many reasons.
It would be easier to find a dealer willing to accept responsibility for collecting all the money and then commissioning an exclusive. But unless they were willing to do so at little or no profit to themselves, I don't know if Spyderco would go along with marking it as a "Spyderco Forum" knife.
The Traditional subforum on Bladeforums has done a forum knife every year since 2006. It has been an industrial pain in the butt every year, and I've learned a lot about the pitfalls involved. I've paid up front for each one since 2007, and another member sold me a 2006 to complete the set. I've actually liked three or four of them enough to carry, the rest are still sitting in their wrappers. On the other hand, I don't have any of the Spyderco Forum knives. Too expensive for something that doesn't interest me.