My reply should be with the disclaimer "Don't quote me on this".
I can see your reasoning to mount all aluminum as it would ease tire rotation and again lower wheel unsprung weight.
There are a couple of issues with mounting another aluminum rim inboard on the dually.
1) As you alluded to in your post, the length of the wheel stud is too short. Yes you could still mount the rim but there would be no threads showing beyond the lug nut, which is not a good thing.
2) Placing an aluminum rim up against another aluminum rim will eventually result in the two rims effectively welding themselves together. Just like steel rims can weld themselves to steel hubs with corrosion, the same happens to aluminum. Could you place some sort of gasket between the two rims? Yes you could, but since your wheel studs are too short to begin with.........
3) I don't know if anyone has found longer wheel studs. You could custom make some but that could be cost prohibitive. The gasket material between the rims would also have to be something that wouldn't collapse or compress as that would lead to wheel nuts coming loose.
This is one of those, I wish I could, but maybe should just stay with what works.
The upside is, I've heard, that the tires are fairly easy to bar off with some technic and if you're using dynabeads, you wouldn't need to rebalance.