Yeah, legally obtained source could help a lot here.
Checklist:
Find out license status
Obtain permissions to use by all IP holders
Obtains source code
Gather interested developers and hand them the sources
Find out possibility that game data could be freewared
Give game copies to all interested developers
Interested developers could be not part of ScummVM Team, but should be willing to join it. Actually most of us have handful of work now, but I know that there are some which could pick up another engine if interested enough.
The guys that work on ScummVM have done a fantastic job and I am sure people, even myself will find games that we wish would run on ScummVM.
I also would love to see TOUCHE' The Adventures of the Fifth Musketeer and Discworld working as they are both classic games, I am sure eventually we will see both, but at the moment there are still so many games that do work on ScummVM, we can't complain really