I have used corner radius endmills as a mini-boring bar to do similar internal hemispheres. You know the corner radius accurately so you can program using tool compensation to get an accurate hemisphere.
The only slightly tricky part is getting the corner of the milling cutter aligned on center.
EDIT
Forgot to mention you might have to grind the opposite cutting edge of the mill back to get clearance but this is quite simple.
An open mind is a virtue...so long as all the common sense has not leaked out.