I would like to project a sketch on to an irregular shaped sheet metal part and perform an extruded cut such that when the sheet metal is flattened, the cut remains.  My problem is best explained when looking at the part file attached.

Currently, the part is shown with an extruded cut which is sort of what I need, but the part won't flatten.  What I tried was to project the sketch on the edge-flange... this is "CompCurve1".  I then tried to use this curve to cut the edge-flange, but this doesn't seem to be supported.

How can I do this?