I Kluge it. I put a construction circle in the sketch (usually circumscribing the slot length), show the sketch in the drawing, put the centermark on the circle, hide the sketch.
It worked in 2004, I haven't had to use it since so I don't know if it still works.
If you use the Slot feature from the Design Library, the dimensions are automatically placed at the centre of the slot. However, the centre is not marked.
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