It's not something i have actually heard of being done in T4 land to be honest hense i to was a bit confused yesterday when you asked i just hoped it was just my lack of knowledge that was the problem alas sadly not so.. If it were me i would join vwvortex and ask them, they are into heavily modifing T4's be sure and come back however to let us know how you get on, i think you will find if you research this matter however that there is absolutely no point as there is plenty of spae for most sizes of wheel on a T4....
Chas - Arch rolling is as Ray say's, mostly used in the racing world to get larger wheels and tyres on a car so as to get better roadholding and better turning circle i would of though illegal ona road going vehicle as wheel/tyre combos have to be INSIDE the arch at ALL times logic tells me if you don't have an arch it wouldn't be legal as teh only raeson for rolling would to put wheels on that wouldn't otherwise fit inside a standard arch 
i would seriously checkout the legalities first chap before attacking your arches ... 
