Maybe get the money upfront?
The ransom model is interesting. Maybe the below variation would get commercial support.
Produce code under contract with the provisio that the code would be GPLed at date X. However, investors get license rights prior to date X understanding, of course, that the code they are using will be GPLed on X. Commercial interests could aggregate their money to get release quality prior to X. The more the commericial money, the longer the time between stable release and X during which the commercial interests have a strategic advantage over non-consortium competitors.
The upshot of it all is that the code gets funded and finished by time X perhaps quicker than it would without the limited monopoly granted to the funders.
Actually, I've seen this work in a slightly different context.