The mono's subframe is a lighter aluminium affair, not meant to take the weight of a pillion. It is also shorter, as I found when I put a (cheapy) mono seat unit on for track day use.
If you were to unlock and lift the pillion pad on the bip and look at the 2 socket head screws at the back that hold the seat unit down to the subframe, you will find it hard to imagine how the bip seat unit can move anywhere.
Having said that .... I know of (and seen myself) one 2003 749 that has a ROUND swingarm pivot hole instead of the usual oval one. Considering that, then I guess there could have been odd variations in frame/subframe design. :P
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