Originally Posted by Nick Jushchyshyn

Here's a PDF tech doc on WSS flagging:
http://www.intersil.com/data/an/an9716.pdf

The flag is a digital signal embedded into the analog carrier on lines 20 and 283 of an NTSC frame so an widescreen TV that detects the signal on a letterboxed 4:3 tape would automatically crop out the black bars.

In theory, this might show up as a visible pattern on lines 20 & 283, so maybe you could capture this of a pre-encoded VHS tape and overlay it into your video? It might even be possible to calculate the pixel values to create an image based on the tech document.

Well, perhaps. Just a thought.
Good luck.