Hi
You can link your overlays with the sequence line clips but, although you can individually lengthen or shorten your overlays by dragging their ends to where you wish them to start and finish, that is, change their duration, they will not automatically re adjust their duration if the clip they are associated with is slowed down or speeded up.
Their position relative to the start of the clip will remain the same. i.e. If the overlay starts 2 seconds into an 8 second clip and the clip is subsequently halved in speed to last 16 seconds, the overlay will still start 2 seconds into the clip and will have the same duration as before.
On the otherhand, if you have linked the overlay and then shortend the clip (speeded it up) so it finishes within the end of the overlay, the overlay will be cut off where the clip ends. If it is followed by another clip then the overlay will play for its full duration into the following clip, but it will still remain linked to the first clip.
I think the only way around it -- if you really want this to occur, although it's long winded and time consuming, is to put your clip with it's overlay on the sequence line and save it as a separate new .avi clip and then load it back into your project to replace itself. If you now adjust its speed the overlay will lengthen accordingly as it will now be part of the avi. It would be probably quicker though to simply lengthen your clips and then adjust each overlay which, as you say, can be a lengthy process if there are a lot of overlays.
Nat