Three track days later I am getting around to installing the OE seat belt buckles for the purpose of maintaining functioning OE seat belts and thus street legality.
Brackets are formed 0.063" 4130 sheet with some turned bushings, a bit of TIG welding, some cadmium plating and some AN5 bolts. They wrap under the mounting brackets. Geometry of driver's and passenger's brackets are not mirror images because the Evo and Circuit seats themselves have different geometry with respect to harness cutouts.
Driver's:
Passenger's:
Some coding is still needed to properly finish the job.