Not the most flexible solution, but if you want the load spec to be authoritative you could allow it to recursively specify a different load generator rather than a task_runner. Instead of running a task, it would start a new load generator.
... hope I got some of the terminology right.
In the example I've modeled each of your uses as a load generator. So you would have to add one load generator per user. Let me know if you have any questions.
For instance, if you wish to represent each user as a load generator, your task_runner's state could store each load generator's user information in the state indexed by name.
You would then log in the user during the init state.
I'm on the subway but I'll describe it more in detail when I get home.