Since the bulk of the replies were posted before the introduction of keyframes, there is now an easier solution available:
- Add a video track, which is on top of every other track.
- Either insert an image or a colour (Open other > Colour) into the track.
- Use the “Size and Position” filter with “Distort” checked to achieve the right shape.
- Set the playhead on the desired end time and press on the clock/timer button to enable advanced keyframes. A keyframe will be generated at this position (Visible in the keyframes panel - View > Keyframes).
- Set the playhead at the start time and type in a smaller value for the height/width. If you now click on play, an animated bar will be visible.
