Wishbone: Well, some of the IP holders are known. Eric Chahi, who made Another World, owns that IP. From [url=http://en.wikipedia.org/wiki/Another_World_(video_game)]Wikipedia[/url]: "
Chahi acquired the rights to Another World's intellectual property from Delphine after they closed down in July 2004."
cogadh: Considering the company was liquidated, there's a good chance the rights to other games are spread over multiple sources. Obviously Chahi is an easy one to find (that's very cool that he was able to get the rights to the game he created), but what about the rest?
As Aliasalpha said, Chahi may know where they went. Otherwise, he probably knows someone who knows it. Finding copyright holders is detective work. You follow one lead, which leads you to another, which leads you to another, and so on. That is one of the major reasons why it is so difficult for GOG to add really old games.
I would have thought that an international database of copyrights existed, akin to the various patent databases in existence. Sadly, this appears not to be the case.