If you really want to go down the path of DE-SEing the DVD, first just take a listen to the soundtracks, whole sections of the films were *completely* re-scored. This is the biggest challenge, the music is completely different, and runs for different lengths of time, and is going to be near impossible to sort out to be seamless for some scenes.

Also, anyone that thinks they are going to drop Laserdisc footage into the DVD release and not have it noticed is nuts. anyone that thinks it won't instantly look like crapola is nuts too.
We are not talking a small drop in quality here, we are talking a major major chasm between the two. You would probably be better off running the old 'super 8' versions through a high end film scanner than using the laserdiscs (lol). Really though, the difference is so great that it is instantly jarring to the viewer.

However you could make the DVD look just as crappy as the laserdiscs and then insert the footage, but what does that gain?

What is possible is with software like combustion and Mokey is to reconstruct and paint out some of the horrible added features using just the DVD as the source, and removing the awful added scenes, and running a bit of colour correction to it, to come up with a 'less offensive' Special edition, but you will never end up with an original trilogy out of it.