Thanks guys,
The idea of using 15" rims actually came from navara.asia ... there are several posts that say it's possible as long as the right offset is selected, but I haven't been able to find any reference to what the right offset is and my two posts on the topic haven't attracted a reply.
Why? Wider range and lower cost tyres.
31x10.5R15 looks to be just right (slightly less than 50mm bigger than standard D22 DX tyres, and only slightly wider than the 265s that are standard on the D22 ST-R).
Thanks,
J.