the famous international dvorak page has the setup files:
however, after the installation, it appeared nothing had changed, and i couldn't select the new keyboard layout in control panel.
after trying to install the keyboard layout dll file manually myself, and after realizing the Microsoft Keyboard Layout Creator was never going to work on windows 8, i finally realized the solution:
simply log off and back on for installation of layouts generated from Microsoft Keyboard Layout Creator to take effect.
since arjen's downloads seem to have been generated using Microsoft Keyboard Layout Creator, it follows that to create a custom layout for windows 8, you can use the Microsoft Keyboard Layout Creator on a windows vista computer to create the setup files, and then do the installation on windows 8.