12 Monkeys could help you with the time traveling idea and Gattaca with the DNA cloning and enhancing idea. Word of mouth definitely would be my strategy of choice, at least to first get it off the ground. You'll need players you can trust that won't get distracted two weeks in and who actually enjoy writing out the plot details - because those interactions can change where plot goes a lot of times.
And, I definitely think at least having a few ideas of what kind of roles you'd like to see filled would be a good guideline for players and also help you control the plot without interfering with players' autonomy too much. I don't know how well the journal-based thing would work, to be honest, though. In these types of games, I always think they get in the way, even because it would be incredibly hard to connect random contemporary characters with people involved with the bulk of plot.
It would be really cool if it focused on the action itself. I can't help but think of The Matrix screenplay a little bit when I imagine how scenes could actually be constructed. What kind of contemporary character spots were you thinking of opening up? Because, I think keeping the focus on the plot would be ideal, since it wouldn't really be a social networking kind of game. Maybe the journals could be used for connecting plot points or some kind of conspiracy theory.
The only way I see them working would be if like, there was an actual plot-related group that worked with them. Like on the new Die Hard movie, the group of hackers that helped create the code, that kind of group that could be involved with actually starting/stopping the disease getting spread, because of the reach of technology these days. I think opening it up for "normal" contemporary characters might make the game lose its focus and, with that much plot twists going on, you probably want to stay completely focused. Plus, you also run the risk of players settling into comfort zones of using only journals to advance plot or getting too lost in the social networking aspect of it, which could make the plot secondary and eventually moot to the other aspects of the game and then you'd just lose a lot of hard work, because people wouldn't be bothered to use the plot points offered by the mods.