I think, since plugin API as very simple, documentation remains enough for that. Just check available structures to get know what feature you may link to.
But, there is no way to control session progress from plugin. However you can generate list of drivers allowed to pass to next qualify.
You also may try to kick out drivers not allowed to do qual. But it is a bit harder: you have to get dedicated server window handle, select driver on driver list and press kick button - all by sending messages to server UI. But note, this is some kind of trick. Don't expect such things described in documentation.
