Winamp's got one over us on this one...
There doesn't seem to be a way to seek through a remote file. Winamp handles this by requesting ranges from the webserver. Obviously if the request returns a valid range, then the seek bar could be updated with the current position determined by this range, and play on from there. If it returns no range, it's obviously not "on-demand", so the scrollbar should be disabled (as it is now.)
But I'm sure you knew that.
Al