It may seem difficult, but we already, in practice, have many classes of fractional persons and it doesn't seem too problematic.

Juveniles, for example, or the handicapped (who have rights in varying degrees - a permanent vegetable doesn't have the right to not be killed by another person, but a low-functioning autistic most certainly does); and past examples of attempts to make someone personier haven't been too bad. (I think here of Terri Schiavo, and the attempts by right-wingers to make her seem less brain-dead than she was - remember Frist's diagnosis-via-videotape?)