How to conveniently separate Cd impurities from ZnO without contaminating it?
Since you actually considered the use of a calutron, I assume that you are interested in separation of relatively small amounts of pure ZnO - on the order of a few grams.

Cadmium and zinc are similar in many respects, which makes me think of rare earths. One of the separation techniques used there is ion exchange chromatography. Resins with special affinity for many ions (either to catch it or reject it) are available. Some contaminants are not easy to remove by conventional ion exchange resins. In many cases, very specific resins have been developed for these contaminants. Selective resins from Rhom and Haas, www.lenntech.com, are available today for the removal of:
• Boron
• Cadmium, mercury and other heavy metals
• Chromate
• Lead
• Nickel
• Nitrate
• Perchlorate
and some other contaminants. 

On the more researchy side, the stability constants of the tetra ammonium complexes of zinc and cadmium differ by a large factor, unlike their complexes with EDTA and similar chelators. For Zn(NH3)4++, the dissociation constant is 9.8 x 10^-10; for Cd(NH3)4++, it is 2.5 x 10^-7. (Reference Book of Inorganic Chemistry, Latimer and Hildebrand, 1951, 3rd ed., pp 135-6). The impure ZnO could be dissolved in NH4Cl (which will also dissolve CdO) and the solution run thru a column. Here's the research: a column of what? Perhaps a column of the impure ZnO. Since the zinc complex is more stable (more soluble), it should concentrate in the first eluate, leaving CdO behind. Solid ZnO would rip NH3 off the cadmium complex and become soluble, so the ZnO will turn into CdO. I have no idea how useful this scheme could be, but the big difference in stability constants is an eye-catcher.