I also looked into this exact thing.
205/30/12 tyres were on these rims and they look hawt, but they would have a rolling diameter so small it wouldn't work.
then i started stalking ride on lawn mowers, and you can get hugely wide twelve inch wheels and turf style tyres. only good for up to about 25 km/h though.
I still looked, but eventually gave up. I conceded that 13's are the way to go, kumho does a 245/50/13 comp tire that would be awesome on a 10/11 but you'd never break traction...
also there's silly widths in commercial van sizes like 225/50 R12.5 that's right, 12 and a half inches. stupid.
I have a pair of those, but they are more than twice as heavy as a stockie.
google atv 12 :.yahoo.jp in images, comes up with some good pics of quads.
if you do go ahead, i want to see pics...