I tried flipping them without spacers and it won't work. the front hits the brake calipers and the rears won't go on flat because of the ring your talking about. they will work with 1.5" spacers. I did not drive it that way though because the front calipers are way out in the open unprotected. and with the rocks here in Arizona I was worried about damage.
I was able to filp my rears by sanding/grinding down the outside tips of the hub so that the wheel would seat flat against the hub. I only did this so I can run my paddles on the stock rims flipped. No way to make the fronts work without spacers though.