I have continued experimenting with the stairways. I managed to find an Intelliscene.cfg for a similar 757 payware a/c and changed the co-ordinates to fit the Freemium 757. Now all the ground vehicles match up with the doors.
However, the catering hangs up when the vehicles are in place, repeatedly requesting "Waiting for your action: Open exit 3". There is no opening doors on the starboard side of this a/c, so I can't respond to the command, and the catering therefore never ends.
Interestingly, with the default A321 and B737, the catering goes on despite closed doors, the attendant just pushing his carts through the fuselage side!
Finally, with the default 747, everything positions far too far back. The baggage loader stands behind the a/c and (un)loads in open air! The single stairway (why only one for a 747 when the 757 gets three?) and the rear catering vehicle drive right through the wing to end up between doors 3 and 4! Pushback, however, positions correctly.