QUOTE
It is done! GSC made its official lawyer-vetted statement:
GSC has made explicit its position regarding the legal status of mods, including the use of its resources with others games. You can find the original under the link below. I am attaching a translation, as well.
http://gsc-game.com/...=6798&sec_id=14
Mods (modifications) of a computer game can be made by a person who has legally acquired a licensed copy of the game and exclusively for the purpose of its use on the hardware belonging to the person performing the modification or persons to whom the modification was given on non-commercial basis (such as freely over the Internet).
This means that the person who makes a mod for a computer game can use this modification exclusively on non-commercial basis.
The author of the mod retains authorship and exclusive rights on the mod. However, the resources used for development of the given mod belong to another person. As such, the mod does not wholly belong to the author of the modification, who does not have full control over it (such as for commercial use).
Our company allows non-commercial use of mods for our computer games. If you want to use mods developed for our games (such as from the S.T.A.L.K.E.R. series) with another game, you have to turn to that game's developers for permission.