You can use Mpeg Advance for quicktime movies, so if you have a working solution for Mpegs using that xtra, you should be able to use the exact same thing for quicktime movies. The QT format is really just MPEG-4 with some custom headers.
But if you really want to use the QT xtra in Director, you would simply have to find the frame rate (I believe there is a .fps or .framerate property) and use sprite("qtSprite").currentTime to set the playhead to the place where you want it.
put QTSprite.movietime -- the current playhead position in the movie
QTSprite.movietime = 100 -- sets the playhead position in the movie (AKA the current frame in the movie)
put QTSprite.movierate -- displays the current play rate of the movie
QTSprite.movierate = 1 -- playing forward at normal speed
QTSprite.movierate = 2 -- playing forward at twice normal speed
QTSprite.movierate = -1 -- playing backwards at normal speed
QTSprite.movierate = 0 -- stopped