Thoughts
Understand the GPL challenge (IMPOSSIBLE): Apparently the GPL's requirement to distribute the modified source code is limited in that you
only have to distribute the source to people that you distribute binaries to.
Debian calls this the "desert island test" [1]. I think the "anti-social hacker" is a better metaphor. An anti-social, introverted hacker, who reads a lot but never posts or sends messages or communicates with anyone, should be able to take your source code and modify it for their own use. This is allowed under the GPL.
Furthermore, if that person distributes a modified version of the binary in a private server, and includes the modified source in that server, they haven't violated the GPL.
What's weird to me, and what I still don't fully understand, is the following: if this hacker privately distributes a copy of the modified binary, but not the source code, they're violating the GPL. But the other people in the server can't sue, because they don't have a copyright claim. The "copyright contract," if you will, is between the original copyright holder and the hacker. So I think the original copyright holder would have to sue the hacker, (in order to force the hacker to distribute the modified source to the other people in the server); but even then, the original copyright holder has no right to see the modified source. (The modified source has to be licensed as GPL of course, so the other people in the server could share it publicly, but they are not obligated to.)