I've used comfrey to heal wounds a few times and it has worked well. these were table saw accidents, which aren't very similar to a pressure ulcer. I didn't bother with making a proper poultice, I just used comfrey leaves as bandages by taping them over my wounds. I also used
honey as an antibiotic ointment. just an anecdote, but it really seemed to help everything heal up very quickly.
I have been told that comfrey can close a wound before the inside is healed. the advice I received was to draw out any infection with epsom salts and arnica before applying the comfrey.