I don't think hydraulic handbrakes are allowed in the UK at present, although a number of car manufacturers are using electric solenoids to activate the parking brake, so I guess they could
be allowed if the hydraulic circuit was independant of the main braking circuit.
Alfasuds, Sprints and 33s have a large forward weight bias, so that when you stand on the brakes the weight transfer is onto the front wheels. So massive brakes at the back will lock up easily, and even if you brake normally the rear brakes will only make a small contribution to the overall stopping power, say 20-30%. That's why the factory introduced a variable bias valve on the rear axle to control the proportion of braking effort that goes to the back brakes. The valve is designed to open up with increasing rear load, so racers remove the valve completely, and most people don't travel four-up anyway. In typical use (single occupant, windy road, pressing on), the bias is towards the front wheels.
The best way of accommodating big disks at the back is to use a brake balance bar, but that requires modifications to the brake pedal, master cylinders, servo system and hydraulic plumbing. Alternatively you can use an adjustable proportioning valve in the rear brake circuit (Tilton make this sort of thing), provided that your brake system is not diagonally split.
There are other, more obscure tricks, like using pads with a smaller surface area, or machining larger discs down in size. Both can be effective at reducing the rear brake efficiency.
You wouldn't think it, but I have a similar issue on the Green Machine - the Alfasud front brakes at the back of the car are too efficient, and even with a balance bar they see so little use they are inclined to sieze up. This issue, together with the always poor handbrake led me to investigate possible alternatives, including Fiat X1/9 rear calipers with adapter plates and cut-down Alfa discs, but alas! My chassis configuration won't allow them to fit