Motion menu: poster frames?

Hi,

I've been trying to come to grips with encore recently. I want to make a motion
menu for a DVD with a video background. The thing is when you link a video file
from the project panel to the background of the menu, Encore links the starting
point to the first bright frame automatically and plays the video to the end.
What I want to do is to select manually the starting and ending frame of the
video file to be played in the motion menu. It is very much like poster frames
which you can use to designate any video strip from the timeline to be played in
motion buttons. Only it seems there is no such option for motion menus, and so
the only way out I see now is to trim the video file in another application,
save it and export back to Encore. But this would lead to loss of quality which
I wouldn't like.

Encore does offer adjustable poster frames for motion menus, but the poster
frame is a property of the chapter.