Projecting a sketch onto a surface, then extruding a cut normal to that surface

Hi, I'm trying to make a cut through a tab that's meant to be welded to a tube.  We're using a waterjet to cut the tab, so the cut itself must be normal to the surface of the tab.  The tube is intersecting the tab at an angle, and because the cut needs to be normal to the surface of the tab, I cannot simply extrude a cut from a sketch of the outline of the tube, as the resulting cut is angled normal to the surface. 

I have tried extruding the surface of the tube and making an intersection curve, but because I then need to delete the tube (because its not part of the tab), I lose the intersection curve.  I have tried projecting a curve and projecting a split line, but because neither of the resulting features are sketches, I cannot extrude a cut with them.  I'm not sure how else to accomplish what I need to do.

SolidworksParts And Features