These are the steps to cross-compile hurd:
mkdir /usr/portage/cross-i686-gnu
ln -s /usr/sys-microkernel/{gnu-headers,mig} /usr/portage/cross-i686-gnu
emerge cross-i686-gnu/gnu-headers
crossdev -t i686-gnu # cross-glibc fails
emerge cross-i686-gnu/mig
crossdev -t i686-gnu # succeeds now
i686-gnu-emerge hurd