Possible? Of course it is. In larger diameter wheels it's common. A quick search on eBay came up with 540 results for a 17x8 with a 40mm offset, which is a 6.1" backspace. Heck a search for 17x8's with a 45mm offset, which is a 6.25" backspace, came back wit 563 results including a bunch of Mustang Bullit rims.
Now, whether or not you can find a 6" backspace on a 15" rim with a 5x4.5" pattern, whole different story. I only found a couple and you wouldn't want any of them on your car...
As for the original question, a 15x8 with 4.5" of backspace isn't going to work very well on the front with a 225/60/15. I ran 225/60/15's on my Duster with 15x7's and 4.25" of backspace and they were very close to rubbing on the fenders. Not just the front corners, but the wheel openings as well. With a 15x8 and BBP you'll have to run a 4x4 ride height to keep them from rubbing with suspension travel. The equivalent backspacing to the 15x7's I ran would be 4.75" on a 15x8, and you'll probably find that you need that 1/4" when you hit the road.
