Really tough. There are two groups of miniatures that could work.
1) Brigade games does british/ANZAC drivers. Can't use these as germans even with headswaps, but maybe russians?
2) Copplestone has pilots upper torsos. Ever seen pics of ww1/rcw russian drivers? All black leather. So these would work, but you have to live without the legs or sculpt some.
Thank you both for your suggestions - which I will certainly follow up. The idea of using the Copplestone pilot figures sounds rather appealing for the use of RCW drivers. Funny enough, I've just found a picture of an Austin-Putilovets armoured car with its crew standing infront of it and they're wearing the black leather....looks rather fetching

However, would the driver be wearing a standard issue cap or something that looks like pilots headgear? (hope that doesn't come across as a silly question).
I also followed up the suggestions made by traveller and I can see the possibilities by using at least two of the RCW artillery crew from Copplestone. Then I thought of Great War Miniatures and checked them out...again, I could use at least two of the WW1 German artillery crew as well.