I don't know how soon, but I may be able to help you in the next couple of weeks if you haven't already found your answer.
I'm looking at a 95 S4 that has a non-airbag wheel on it. The seller said the previous owner made the swap. He also has a 90 SE w/airbag, but he doesn't want to keep it. If I'm buying, he's proposing to take the airbag wheel off the 90 SE and put it in the 95 S4. He just doesn't know what the guy did who swapped it in the 95 S4, so he's going to have to see if he has all the parts. I would have thought taking the airbag wheel from one car to the other would be very easy. I'm hoping you're suspicion is right that the splines are different on the UK or ROW cars, but are the same on all the SRS Fed cars.
I'll let you know what I find out, if you haven't figured it all out before then.
Interesting, do you have any pics of the wheel on the S4 ?