Did you ever see Downtown Anywhere?
I think that was its name.
Basically it was like a "Choose Your Own Adventure" book, where the different choices that led to changes in the story were written by different people, and more could be added at any time.
It was kind of like the hygrid in structure I guess, although a story can have more than four sides. =)
I don't know if it's still out there.
What's going on with Impulse Freak? I need to exercise my pen.