My machine spins up to 1200rpm. I don't think an endmill would work.Can you rotate the work piece 90 degrees so the face where you want the slot is facing the spindle and cut the slot with an end mill in a holder or collet? That's what I would most likely try...
Or, if you need the slot facing up for some reason, can you make a fly cutter out of a HSS tool bit to cut the slot? You might need to make a holder for it that will fit on your bar. And, you would have to feed it slow, but it should work in a pinch.
Just a couple of thoughts to throw at you,