Why stop with the symphony's model when you can go all the way back to the composer's model? Tell your patrons / investors / early adopters that you'll work their ideas or their characters into the material. Use their character concepts for illustrations, or retell episodes from their campaign to illustrate mechanics, or have their characters as kickass NPCs in one of your settings. If they want to invest up front, you can put them in from the start; if they want to buy it after it's complete, you can work them into a later version, perhaps also including excerpts or suggestions from when they played with the material.

If your shout-outs to paying customers won't be added until later, that also gives them an incentive not to leak the material early. If the public release is going to tell all about about the totally awesome campaign you and your friends ran, that's the version you'll want the world to have!