The problem with the BMW poly bushes is they are designed to be over-length and when the centre bolt is tightened this causes the end cap washers to compress the bush making it a tight fit in the outer sleeve.....if you want a stiffer bush you just make a longer one and compress it more.
On the Omega the bush has to be a fixed length because it fits into a defined gap in the front subframe, there is no way that I can see of compressing a poly bush to get it into the gap.
The ones we found in the states were a moulded poly bush and would fit but I cant get a reply from the makers.
BMW M5 and 750iL bushes will fit (mentioned on posts from someone in Taiwan or HK) BUT......the centre hole is not concentric with the outer diameter, this changes the front geometry too much. I tried them for 12 months and they really sharpened up the steering response but at the expense of a pair of tyres.
BMW E34 bushes also fit but these are the same as the latest genuine Vx bushes. (note I think some pattern wishbones use the earlier Vx bush which is softer).
If we find a source of better bushes we will post on here, may even be able to sell them through the shop or offer a re-bushing service.