At the rear, you are talking trailing arms, and the castor correction only really applies to the front...
However, you have a couple of options - Cranked Trailing arms will relieve the angle at the chassis end, meaning the bush has a much easier time, and also means the range of movement is not limited by the bush.
I have these:
Another option is the x-eng x-flex system which replaces the standard bushes with High movement ball joints.
Incidentally, knackered bushes give more movement than good ones at the expense of road handling and comfort...
As a rule, you get one or the other
hth
Mark