The code owned by the company could be released without those proprietary libraries (or code, if embedded directly). The game fans would then need make/source a drop-in replacement or remove the need (if the 3rd party code is something like DRM then they'll do the latter). This is of course work, but less work than replacing the whole server-side perhaps making it at least practical.
Of course it might not even be as easy as that, depending on how the code itself was contracted out and how the rights were distributed around parts of a larger company that might have since split & merged a few times, it could be difficult to make sure such a release only contained what it legally can.
Of course it might not even be as easy as that, depending on how the code itself was contracted out and how the rights were distributed around parts of a larger company that might have since split & merged a few times, it could be difficult to make sure such a release only contained what it legally can.