To help with mass installs at our party, I wrote a simple installer for RockBox in Python that would run on GNU/Linux. With a few modifications, the script could be relatively easily converted to also run on Mac OS X.
While an unofficial installer exists for Windows, my installer, simply called the iRony RockBox Installer, seems to be the first installer for GNU/Linux.
Of course, the installer is still largely rough and untested and is definitely unsupported by the RockBox team. If you get around to trying it, please send me feedback, advice, or and patches. You can grab it (and future version) here: http://mako.cc/projects/irony/