I don't think it would work well. I do machining at my work. You would have to put the mill bit so far down to get a slot that it would chew up a good piece of the filler as well. Unless you could machine it on a horizontal mounted chuck of some sort(like from a lathe) and rotate it on the mill table. But we don't have that setup. We have a lathe and a mill but nothing to use a rotating lathe chuck ON the mill. Theoretically, that could work well I guess.
I do dot my cartos using a 1/16" mill bit though. It goes
through like butter with much better control and leaves a much better finish than a drill bit.