That is a very good question Juan.
I've honestly never really thought about trying to improve the rear suspension in that way.
On the face of it, allround fully independant suspension is probably the ultimate setup but I don't know whether it'd make much of an improvement on the Polo mk1-3 in real terms.
TBH, the standard rear end layout is very good. The damper/springs are mounted vertically and right behind the hub. This means spring rates can be kept quite low and the dampers can work as intended. The beam's mounting bushes are of a pretty good size too and the mounting points are in strong areas under the car. If only as much thought had been put into the front suspension design we'd all be laughing.
The stresses sent through to the rear of the car are minimal - with the car being so light, suspension travel is fairly limited in use.
As you say, it certainly would be possible to fabricate an independant setup for the rear but in practise I don't think it would serve much of an improvement.