By racing belts I assume you mean a 4- or 6-point harness. These would make access to the rear even more difficult, assuming you could find a secure place to mount them correctly, as they need to ideally go horizontally back from the drivers seat to their mounting points. You'd also need a drivers seat designed for a harness with holes in the back through which the belts should run. You should never attach the harness directly to the seat, as it is fundamentally NOT designed for this and it would be dangerous in the extreme.
It might be easier to simply get into the back from the other side!