I will probably not have the time to answer longer this evening, so I'll do a short answer for now: yes, you can use the iopl device instead of mem to get the expected behavior. GNU/Hurd needs a /dev/mem as it is a very commonly used interface, not only by Xorg/pciaccess, but also other low-level hardware tools. Using the iopl device is a bit of a hack, a more proper solution needs to be done. Maybe in the meanwhile we should alias mem and iopl in the gnumach kernel, so already installed hurds can run Xorg (and the name "iopl" doesn't sound so good to me to replace mem :)).
Samuel
