Tony - I have just had the rear cross tube replaced on old 206 by my local engineering shop at a cost of $A1500. The car was in dock for a day and the guys doing the work had no trouble cutting the old one out and fabricating a replacement. There was no corrosion on any other parts of the rear subframe. So, as you say, if you just have to "fix it", it is actually not that hard to do. Don't forget the lateral links while you are at it. They are much more scary...
Some photos of the recent work attached.
Malcolm