I have done something like this before - each poster just adding a piece of the story, generally each from a single protagonist's perspective. We called those 'serial stories' rather than roleplays, and that was also where my history of collaborate writing began. To add to the insanity, everything was purely jump-in (granted, it was a site roughly tenth of RPGs size, and it wasn't dedicated to RP). We managed to successfully finish a handful of them, too, so it wasn't as disastrous as one could expect. 
    Ehh, nostalgia... The site eventually died due to owner neglect, and the site's successor (created by former mods on the site) is now I think eight years old already. (It has to be summer '07 or '06 when the new site was created.)

   But enough of my rambling; I just stepped by due to how oddly familiar to my ages past this all sounded. It'd be an interesting experiment, but regrettably I'm a bit overwhelmed with real life work, so I can't really commit to more RPs but two. Best of luck to you!