I have been power tapping in a mill using a drill chuck for many years. If I need to do a lot of holes then I use a tapping head. The largest tap I have used under power in a mill is 1/2-13 in aluminum, the smallest is 10-24. Use proper cutting fluid, turn slow normally in back gear, use sharp taps, and use spiral point taps where possible.
Many times I will also start the tap under power and finish by hand, especially in blind holes. With small taps, I may put the head in neutral, and start the tap by hand by turning the chuck, the release the chuck and finish with a tap wrench.
EDIT: I should read the WHOLE title of the thread before I post. The above is only valid if you have manual control of the quill.
There are floating tapping chucks that can be used in a non-synchronized CNC. Without knowing the rotational position of the spindle relative to the Z position it would be virtually impossible to power tap under CNC control with a ridged setup.