I thought I'd try a little experiment! I knew you could grow comfrey from root cuttings, crowns, or seed (if you got the viable variety), but I wanted to see if I could propagate from the leaf stems. My comfrey had gone to flower, so I wanted to hack it back anyway. I cut some of the fattest stems into sections about 2 inches long and stuffed them into seed raising mix. I sliced some sections in half lengthways to see if that worked, too. It worked! The ones cut in half took longer to germinate, but it worked. The only sections that haven't germinated yet are the stems I completely buried in the seed raising mix.
If there's anyone out there, like me, who doesn't really want to dig up their plants, give stem cuttings a go.