Just me others might want to, but I wouldn't not after looking at what your talking about
https://www.google.com/search?q=Stromberg+Carlson+TR-1+bumper+mounted+carrier&source=lnms&tbm=isch&sa=X&ei=q-xpUtfRG9HSiAKpyYH4DA&ved=0CAcQ_AUoAQ&biw=1366&bih=620 maybe a small trailer tire with a good strong hitch. I think your tire weighs maybe 80 pounds or more and it is going to be mounted at the very end of the "back of the bus" bounce wiggling around. If you tow, I would get it up front with a good strong front hitch (square tube) If you don't tow, mount it in rear hitch. IMO
You said "I'm a concerned it may not snug down properly because of the dually wheel." Yes, good catch with the offset wheel and bolt keeping it on. Mine only goes on one way (inset to rack)and four-way wrench bottoms out on tire before nut is tight. I just use an extension to get bolt tight. I will say that is not the worst place to carry a spare tire. Saw one going up freeway last week with spare tire chained to luggage rack on top of RV just bouncing along on the roof.
edited part: I like something like this although I would like it even more sturdy.
http://www.etrailer.com/Trailer-Cargo-Control/Curt/c31006.html