It would be practical if each slider has 3 events it can send out: startsliding, sliding, stopsliding (or comparible names). Currently the only "event" we have is stopsliding, which can be read using a hook to mimic a 'sliding' event.
From what Seb says, I don't think that's quite true.
In polled mode (using WaitEvent), you're right, there is no extra event. But in when using EventHooks and WaitSystem, it seems that the events do come through.
Which still seems bizarre to me, but hey, that'll be Microsoft...