With the help of a tip by Darren Dunham, I found the right way to boot an SVM mirrored system in single user mode (GRUB's default Failsafe configuration ignores mirrors during boot, and there's apparently no way to mount a mirror).

When GRUB menu appears, hit 'e'
Select the 'kernel ...' line
Add 'kernel/unix -sv' to the end of it, so it reads:
kernel /platform/i86pc/multiboot kernel/unix -sv
Hit 'b' to boot.

It should boot into single user mode (with mirrored / slice mounted, etc).