When flipping to under table shaping, the shaper cutter is now inverted also(upside down). That is, what was the bottom(table) end of the shaper is now the top end and vice versa.
If the table/cutter positioning does not matter, then the rotation does not either. With straight bit routing it does not matter either.
This is not the complete tale however. A shaper fence is also being repositioned relative to the cutter. From the operators perspective, it is still the same, but relative to the headstock and it's rotation flips.
Having the ability to reverse rotation allows mounting the shaper cutter either orientation.
HOWEVER: One must keep track of the proper direction of feed. When the cutter is reversed(either on the arbor or by flipping the table) the feed direction is also reversed. i.e. plow vs climb cutting. Plow cutting pushes the workpiece against the fence(OK). Climb cutting pulls it away from the fence(not good!!!!!).
My confusion re parts mix has to do with the 'meaning' of 'base'. To me the MVII base is not the part containing the way tubes etc.
From your latest pix, I think you are wanting to mount a Mark 5/V way/bench tubes and end caps on a 'bench' and use the 'normal' pivot for drill press use, but add the ability to tilt the other direction for under table shaping etc. that pivots similar to the MVII that places the headstock and table lower. That would mean a short 'bench' with pivots like a MVII on the end opposite the 'normal' M5/V pivot.
I think that could be done(IIUC), but the release for the 'normal' pivot end would would need to be created. The Mark 7 end cap does that. A Mark 5/V end cap can be modified similarly(done that already;)). A Mark VII type pivot/catch need be created for the other end.
Damn this is intriguing!!!
Interesting idea! Eliminates a Mark VII shortcoming(drill press too low:D).