Hi Pete
Please be aware that i am new to CNC and do not (as yet) have a machine so i am approaching this from a different perspective. If a suitable taper cutter is available could you machine the pocket to it's base dimension with a parallel cutter, change the cutter over to the tater cutter,set the cutter just cleare of the bottom of the pocket and incrementally step the cutter out until the the side is finished(no steps)
Plan "B" again you need a cutter with the required angle, just make a router template jig and do it by hand,obviously producing the oval shape is the only hard part to the job, but it's easy peasy on your CNC.
Does that make any sense.
Regards
Mike