This is not a new idea, but a very old one. When companies first started to be funded by people unconnected with the management (railroads for example), they generally had classes of stock that offered some prespecified return.

Funny, because if you read the Software Bill of Rights (http://softwarebillofrights.org/license.html), that's pretty much what we do: a cut of revenue share while the company is only making money, but convertible into equity if the company gets bought. The only dimension we don't have (because the SEC wouldn't like it) is the investment part. So currently, you can only earn those class "R" stock if yourself contribute sweat, not cash. I'd love to fix that hole though...

Isn't this what a convertible note is? Admittedly that's restricted to qualified investors, and is not tradeable on the open market, but this doesn't sound too unusual.