I think that you will find that for registration with a disc/drum combination a dual circuit master cylinder will be required.

You also keep refering to a single resevoir, but some dual circuit master cylinders use a combined single resevoir & some use two separate resevoirs, which is why I asked about that. The number of resevoirs, in this case means diddly squat.

Disc brakes, as are fitted to the front of this car, require no residual pressure in the brake line when the brake is released so that the caliper can release all brake pressure on the rotor, but drum brakes require a small amount of residual pressure in the line to ensure that the cups in the wheel cylinders remain in contact with the walls of the cylinder at all times to prevent the entry of air. The strong springs on the brake shoes will ensure that the brakes are fully released.
With a single circuit cylinder you will either have residual line pressure, which is bad for discs, or no residual line pressure, which is bad for drums, both of which this car has.

That's why a dual circuit master cylinder is required. That's why every manufacturer that uses a disc drum system, even before the ADR's required it, utilised dual circuit master cylinders, yet you say that you will front for a blue slip & registration with a single circuit one.

Good old Professor Julias Sumner Miller unknowingly passed to me the best gift I ever received, ..... a question.
Whenever you see something that you have not seen before, like this for example, or when pressing a spongy brake pedal while watching air bubbles rise in the resevoir, just ask yourself 'why is it so?' There will always be a reason, we all just need to figure it out.