I'll rise to the bait
I've put drills in my mill, so in this case if you have a drillpress I'd not hesitate to put an endmill (with some kind of an adaptor) in the drillpress.
Go easy and clean out often. Make sure you get a 4 flute endmill. That will give you a very nice flat bottomed hole.
I'm not an authority on flux leakage, but I think it may be less than you'd think by the simulation pictures, your airgap will be very very small by doing it the way you suggest, assuming you keep your holes just deep enough. A compromise that would work even better would be to turn 1/8 of an inch off the surface of the rotor, then drill the holes.
that way you get some sideways support to avoid the magnets moving due to forces acting on them when you start making power, and you'd get rid of some of that flux leakage, which is always better than having it, no matter how much effect there will be.
Adapting an endmill for use in a drill press is easy, you can either ask a machinist to turn it down for you to a size that your drillpress can handle (most drillpresses will do 1/2" bits, but check to be sure), or you can make a collect holder for your drill press. Some drill presses have replaceable chucks, with a taper, and you should be able to find a collect holder for that taper.
do wear those safety glasses