The low pinion diff spins in the correct direction, it just spins on the coast side of the gears. If you flipped it upside down, you would (like you said) be running backwards. To counter act this, you would need to redesign the T-Case to add either another intermediate gear between the drive gears on the front and rear outputs, or eliminate the one that's there currently, and figure out how to directly link the two output drive gears. Maybe adapt some sort of chain/belt drive. This would make your front driveshaft spin the way you want, but would be significantly weaker than stock (IMHO).
You could redesin the front diff, so that the carrier would flipped to the opposite side of the housing. So the housing would stay upside down, but the ring gear would be back into its original position. The reason you would have to redesign the housing, is because the pinion gear is not centered. It's offset, and there isn't enough clearance for the ring gear on the other side of the diff. Once you got done, you would have essentially designed the highpinion diff already in existance.
You could contract on a major gear manufacturer to design and cut you a set of one-off reverse rotation gears. "IF" they would even do such a one time thing, the cost would seriously outweigh the cost of a highpinion diff.
Or, you could turn the entire drive line around backwards (including flipping your rear diff as well), putting your engine in the back, having your tranny face backwards, and essentially moving your rear output to the front. You will have 1 forward gear, and several backward gears. This I would like to see, because I think it would be hilarious to see someone doin 60 MPH backwards to and from the trail head. Hell I'd ride in it just for grins. But to do it properly though you would need to figure out someway to flip the gear sets around. This will also either require some interesting driveline work, or else retubing your Axle housings (front out put will now be in the center, and rear will be offset). It's a difficult concept to grasp (for me atleast), and it took several friends explaining it to me with a vehicle set-up like this in front of me, to fully understand it.
Then there's the oiling issue. The highpinion diff is designed with an oil slinger washer that dips into a cup to oil the pinion bearings. This cup is in the casting. This cup doesn't exist on the v6 housings. So I would imagine you would have issues with keepeing all of your bearings happy. You could overfill the diffs, but no guarantees that they would last very long as daily driven diffs, or even bombing around in the desert.
Now all of this would cost insane amounts of money, and a highpinion would be a hell of a lot cheaper. Or (as previously stated), you can run low pinion stuff, and see how long it takes to break. You might be surprised.
