I use the original plastic air cleaner too, and do a lot of dirt roads, although not as many K's as the outback tourers on here.
Considering the amount of dirt roads I do, I was concerned about it too and I looked into it not long after getting the Moke on the road.
What I found was that there are good and bad examples of 40 year old plastic air cleaner housings. I found a good one that clips together securely at the top and has good seals, then for good measure I use a K&N oiled reusable filter element. I don't get any dust on the carby and very happy with the set up. I personally think the Donaldson cleaners are overkill unless you are doing outback trips involving thousands of km on dirt including powdery bulldust outback roads.
The best prevention of all of course, is to stay out of the dust where possible - ie. don't sit in a dust cloud behind the vehicle you're following. If you're regularly doing dirt roads, you probably already know this, but it's something we notice on our Tamworth Round Up runs - it's amazing how many people spend their days choking themselves, their passengers and their vehicles on dust. Some dust is inevitable when travelling dirt roads, but it can certainly be minimised.