How difficult is it to port the GNU/Hurd system to run on another architecture?
The GNU/Hurd system consists of Hurd servers running as user-space processes on top of the GNU Mach microkernel. The system functionality is usually accessed through the POSIX interface that is provided by glibc and libpthread.
A whole-system port involves touching all these components, with varying degree, of course.
For a CPU architecture port, the microkernel is the most involved part, followed by glibc and the threading library.
The original Mach microkernel was portable to a number of architectures which were a lot more popular at the beginning of the 1990s than they are now.
Further on, there are some unfinished porting attempts for the Alpha, MIPS and PowerPC architectures. These have not been completed due to little developer interest.
Another option is to do the port at a different layer: port the Hurd servers to not run on the GNU Mach microkernel, but instead on top of another microkernel. Or, even by providing a Mach emulation layer on top of a monolithic kernel. For example, there could be a port for having Mach run as a POSIX user-space process, or by implementing the Mach IPC facility (as well as several others) as Linux kernel modules. While there have been some experiments, no such port has been completed yet.