Nowhere in the regulations does it specifically state passenger seating.
They were thinking about the pilots though....
Sec. 23.771
Pilot compartment.
For each pilot compartment--
..... and
(c) The aerodynamic controls listed in Sec. 23.779, excluding cables and control rods, must be located with respect to the propellers so that no part of the pilot or the controls lies in the region between the plane of rotation of any inboard propeller and the surface generated by a line passing through the center of the propeller hub making an angle of 5° forward or aft of the plane of rotation of the propeller.]