Do 12inchers fit over the sigma brakes? I thought it were too tight. If you put your offset further in toward the vehicle (on the inside of the centreline) doesn't that mean you are adding width to the tread but not creating any dish? You're creating positive offset correct? I think you need to go the opposite direction to create more negative offset.
 
I think getting your wheels banded and widened can be a little expensive, and you can actually get 13 x 6 factory wheels (from a 510??). I haven't checked for a long time but I think the last time I did check it was going to cost $150.00 per wheel to have it widened. I'm not sure if I had to supply sacrificial wheels too.
 
Personally if I were to get a set widened, I would be using 13 inchers and I'd be making good use of the costs and getting them as wide as possible 13 x 8 or more.
 
When you say the old school deep dish look are you talking Japanese stretched rubber style? If so you might want to consider the tyre profiles available for the 12 inch series. Dish and big puffy swollen rubber looks a bit odd, unless you're pursueing that look. There was a bloke in perth who owned a red wagon with watanabe's, he had the tyre profile completely wrong and it (in my opinion) ruined the look.

That's a good point... I never even considered that the 12's might not fit over the calipers and you're right; they don't!  Thanks for correcting the +ve/-ve offset confusion too, always get them mixed up! 
 
The tyres I wanted to run with are a low profile (http://www.bobjane.com.au/tyreset-details/tyresetID/231/idm/12/ispt/Tyres/isu/6.html?sid=e1b7fd4aaf2e513b), which aparrently would have stretched to fit a-la Tokyo Drift Boy Racer style. Price wise, I work in the auto industry so widening a set of 4 rims and putting new tyres on them would've only come to just under $1200... Considerably cheaper than a new set of Watanabes and new tyres to suit. Oh well, I do effing love those wheels on a 1000, looks like I'll have to start saving.
 
Well, looks like I now have a set of 4Jx12" rims with tyres spare and a full set of 1K wheel trims. I may still widen them and get the new tyres, so if anyone is still running drums and would like a set of 12's that no one else has (been mad enough to have) done, let me know!