Minecraft is *not* supposed to download LWJGL or any .jar files on Gentoo. It just uses the system libraries insted of using its own.
Apparently that makes Forge just stand there dumbfounded not sure how to react. It seems to depend on all jars being shipped along with the game instead of being system-wide libraries.
/usr/games/bin/minecraft on Gentoo -- works perfectly with Vanilla, but Forge cannot compute