As a current MOT tester since 1989 I can add the following

When a vehicle is presented with "clearly" no rear seats or seat belts (ie stripped or modified) then that in itself is not an RFR (Reason for Refusal).

As for your replacing of front belts with harnesses then again this is fine

BUT

The method used to secure the harnesses WILL be assessed by the MOT tester - corrosion and security of body or affected panels within 30cm radius of any harness fixings.

The suitability of the fixings used -(So dont put a 6mm bolt when it should be a more substantial fixing)

The metal chosen to attach the harness fixings to - Flimsy rear parcel shelf and similar WILL FAIL

If you attach any part of the harness to a roll cage then immediately ALL fixings of the roll cage and its condition and suitability become part of the areas assessed

A lot of harnesses use the original seat belt mounting captive nuts - thats fine

Then as per any "normal" seatbelt the condition of the harness webbing, locking and release mechanism, mounting eyes etc etc etc will all be assesed

Hope this helps