Dermatophags (wolf-biters) eat the skin around their fingernails, similar to how some people chew on their fingernails. Can one do so on Friday?
I would have to answer yes, provided you do some other sort of penance. The last time I checked, the obligation to abstain from meat on Fridays may be substituted with another form of penance.
Always ask your confessor or check with a diocesan authority when you have questions on Catholic discipline.