How can you claim the restraint approved for infants and small children applies here?

"The FAA has approved one harness-type restraint appropriate for children weighing between 22 and 44 pounds."

Indeed, the FAR does not specify minimums for seats used for medical reasons:

"This paragraph does not apply to seats on which cargo or persons who are unable to sit erect for a medical reason are carried in accordance with procedures in the certificate holder's manual if the seat back does not obstruct any passenger's access to the aisle or to any emergency exit."