Actually I've slightly modified the game sourcecode, so that there are now 2 addon folders loaded. One in the .opk filesystem, the other one in ~/.uqm/addons. So, even if some content is included in the .opk, the user is free to add additional official content files via ~/.uqm/addons.
Regarding the copyright: I've already put the game content in the .opk, which is unter the same license as the official addons, so if there are issues (I think there aren't), they are not effected by adding or removing addons from the .opk.
@Gab1975: I like the idea of two versions.