Okay, at least for this application I finally got it. The trick was to use SLICE two times using the same line along the horizontal to vertical tube interserction to slice the diagonal tube. The other part of the trick was to adjust the UCS accordingly before making the second slice. But it works and now the guys in the shop will be even more confused, because they want to just cut it square and fill in the gap with weld. Now you see how difficult it is when the rubber really meets the road.