A further thought towards making an approximation to structure weight:
Structure weight = empty weight – (weight x number of engines) – (weight x number of seats)

Presumably engine weights are published, and a standard weight could be used for all seats?

Is there anything else substantial that should be deducted from the empty weight to get structure weight only? Older aircraft tend to have removable fuel tanks, whereas more modern aircraft have the tanks as part of the structure, so not sure how to handle that.

Thanks for any comments on these.

Iain.