I remember reading on the SCUMMVM boards when they were first released, that the files are fairly easy to modify to work with ScummVM, it's just that most people won't release publicly how to do it because of legal issues. If you ask or search on their boards you might get the info via PM or email.

Not much to do with legal reasons. It'd be trivial to make ScummVM compatible, but the developers don't want to interfere with sales or any crossplatform releases LucasArts might make. They basically don't want to do anything to upset LucasArts, even though they'd be in the clear legally. Developer post

The DRM itself doesn't have to be released as open source, but "hooks" added to the GPL program's code would. Regardless, any degree of protection added to emulated titles is always going to be paper-thin (even Wii Virtual Console ROMs can be extracted with the right tools) so no company is going to waste too much time on it (especially since Steam itself offers a basic level of DRM anyway).