Frame integrity is quite a big thing, especially on an altered frame rather than one designed from scratch to be split. There is also the limited life of the Gates belt, which only recently was said to be as low as 5000m (or was that 5000km?); the Dutch tourers you quote have already done much better than that. The question stands though: if a Gates belt were to give up the ghost in the back of beyond, could they change it themselves or are special tools necessary? (I don't know but it is a question worth answering.)
But it is your bike. You could ask Thorn if they will alter the frame and fit a Gates drive for you. If they won't, you can get someone else to do it. Perhaps the chap who brazed all the steel Thorn frames until a few years ago will do the job for you. Maybe Bob Jackson can help you; they're very easy to deal with. In either case, the job will be done competently. It's likely to be expensive, because you'll need a repaint.
If you do it, don't forget to report here. We'd be keen to know how it goes.
