Andrew,
What you said about fitting the heaviest possible engine corresponds to my experiences. With a 2-stroke engine one would indeed have a bit of margin, and keeping the weight of the occupants within limits is a worthwhile exercise. OTOH 2-strokes have a certain reputation of becoming unreliable if not cared for very intensively - always seemed a bit feminine to me (ducking). I had my own grand moment of loosing power on take-off and toppling over less than a minute later behind a Rotax 2-stroke, it's hard to shake off bad memories.
And yes, certainly (bathroom) scales will have the final word to say vs. the POH.
One word of yours puzzles me, though: is there really such a thing as an "authorised empty weight" ?