Just played around with some drawings on Solidworks. 5x4.5 to 6x5.5 is not possible with a single disc. 6x5.5 to 8x6.5 is also not possible. This is all figured using 1/2" studs and 3/4" lugnuts for approximation. If you eliminate the space used up by lugnuts and sockets, either are possible, but not practical. You would have to have a 2piece design, or a way to attach the adapter to the hub without lugs.
Any equal number lug pattern is possible (5x4.5 to 5x5.5, etc), and 5 lug to 8 lug is possible due to pattern diameter difference.
After some more thinking, it could be done with about a 2" minimum thickness, and the use of lug bolts instead of lug nuts on the wheel pattern. However, a lot more machining would be required to pocket out the areas for the axle pattern lug nuts, increasing cost some more.