It's impossible to play on public Cube servers with custom client. Here is patch for ebuild that adds USE flag 'emul' to select if you want to force instalation of this x86 binary (which works just fine on amd64 with emul packages)
Created attachment 75473 [details, diff] patch for installing prebuilt cube binary on amd64
Due to cube being masked for security reasons, we probably won't ever do this unless upstream releases a new version. Sorry... =[