If you can have a pivot hole, you can set up a pivot pin on the milling machine table with a nut to hold the piece down, and swing the workpiece AGAINST THE CUTTER ROTATION by hand. Be doggone sure you don't do anything remotely resembling climb milling though, or the workpiece will climb right out of -- or through -- your hand and cause general havoc. I'm not sure I would try this stunt in a closed slot.
For a closed slot, set up the pivot pin etc. but drill closely-spaced holes along the curved slot, then clean out the webs in between the holes with plunge cuts with a center-cutting end mill. Then clean up further, if necessary, with further plunge cuts, CLAMPING THE WORK DOWN IN ALL CASES. When the curve is pretty smooth, touch it up with a file to take off the last humps.
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