pressurise a poptop van to exclude dust,

A snorkel as you suggest the problem is you'll need to get it above the dust stream
generated by the car.

Gary , it depends where it gets in, is it in the floor cupboards?We have a small pop top and when towing it to Mt Dare it got a lot of dust inside. I spent a few hours completely sealing the under side perimeter with flexible sealant so nothing gets in the floor areas, that stopped most of it. check all window seals, surprising how much flex is there, also the roof seal and door seal. We travel with the small roof vent open, and except for passing traffic very little gets in there so a snorkel type vent may work well. good luck Keith.

A few vans are now coming standard with pressurising fans.

I made one up which was very similar to the one the manufacturer uses.

A box made from ply, a bilge fan from a boat shop, a filter from super cheap, 2 vents ( one inside and one outside ) and a bit of flexible hose.

Mount the outside vent up reasonably high on the side of the van, the fan then sucks air in, thru the filter and expels into the van.....which pressurises the van..

Doing this mod whether electric driven or wind the components won't prevent dust coming unless you have more air coming than going out. Once your camper reaches a speed of 30km plus it acts as a vacuum.
I'm old school and like non mechanical components because they cost you to run, can break down when you need it most and expensive to fix but they're all designed to do the same thing once you have prepared the camper.
I only sealed the fridge vents & door vent with removable covers, all pipes entering through the floor with flexible sealant, flexible waterproof spray the underneath of camper and a rough adhesive strip on camper body where roof rubber meets to get a better seal and a snorkel oiled filter sock with another 2 filters inside the camper, thats it.
Worked 100%, easy, fairly cheap with with no mechanical moving parts.

We have a Jayco Poptop caravan. It has a double door - a 'security' door and an outer door. There is a large vent in the lower part of the outer door, designed to let any leaked LPG out of the van (LPG is heavier than air). We block this vent off whilst travelling on dusty roads with a piece of blue foam carpet underlay, simply placed between the inner and outer doors. That blocked I would say 95% or more of the dust when we went from Nocundra to Tiboorburra. At times, I could not see the van for dust - it was horrendous. Very little dust got in, and that which did entered under one of the beds at the back of the van. Since then I have installed a scoop type vent in the roof near the front, but sadly we haven't been back out to Tiboorburra to try it out. Suffice to say that my wife is quite happy with just the foam stopper in the door (AND SHE IS FUSSY!).
If you do this mod, always make sure that you take the foam out from between the doors when you stop and set up because it is a gas safety vent which should not be blocked.

Have a look at this Youtube clip to see one solution.

https://www.youtube.com/watch?v=zzM_Lq9Lp10