An airplane has its right front seat belt broken. Is it legal to fly,
assuming nobody is sitting in the right front seat? I say no, others
say yes.

The reason I say no is that 14 CFR 91.213 (d)(2)(i) says that it is only
legal to fly with inoperative equipment if the that equipment is not
part of the VFR day type certification. I believe that seatbelts ARE
required for type certification by Part 23 (or its CAA predecessor).

Others say it is ok to defer to the next regularly scheduled inspection,
as long as nobody sits in the seat with the broken seatbelt.

Who's right?