Some SR22 have a useful load of over 1100 if not equipped with ice protection. I know new planes can have 5 belts, not sure if you could retrofit.
I was sure you were wrong.
Assume the seat mount is the same a retrofit would be a easy sign off.
Cirrus Launches Five-Seat SR22
At its Global Sales meeting at Disney’s Epcot Center in Florida, Cirrus
unveiled the latest Cirrus SR22
to its dealers, and the big news is, it now has an optional fifth seat. The five-seat layout is not a new model but rather an optional seating layout for the 2012 SR22, so there’s no new model name. Cirrus achieved the additional room needed for the added occupant in back not by modifying the design of the airframe but by changing the profile of the side panels, creating several inches of extra room.
With the optional seating arrangement, the airplane can now accommodate a fifth passenger on a split bench seat. Cirrus is not touting it as being large enough for three full-sized adults, and there are no changes to the operating weights of the airplane. The center belted seat is narrower and is a good fit for a small child between adults or for a group of youngsters.