Here is a project I came across that is a new take on the rotary axis.  This uses a rack and pinion to rotate a part when the Y axis moves, basically turning y motion into rotary motion.  This is similar to the belt driven solution I posted earlier for lasers but this is ridged enough to use a spindle with.

Here is another thread on a similar project.  They are programming the mill by drawing up 2D paths, so for example they draw the profile of a bat on the XZ plane and extrude it horizontally, then wrap that to fit the rotary axis.  In there case there rotary is driven by the Y axis so instead they need to do a bit of math to scale things correctly and just machine it as if it was a regular XYZ machine.

Anyway the point here is that I'm overthinking things a bit.  Yes it would be nice to model a 3D part and have it cut out perfectly on the rotary axis without modification.  But it is possible to get some complex shapes out of the rotary using 2D cad (2.5D) and some creativity with the math.