Since I can't get answers elsewhere, I thought I'd try here.
I believe the original plans were as follows.
For items that
DO NOT
have item XP, you will get an extra 50% Invention XP when you disassemble them.
(for example, an Addy sword)
For items that
DO
have item XP, you will not get any extra Invention XP when disassembling them.
For items that
DO
have item XP, they will gain Item XP at a rate of 50% more than normal, when using them on Double XP Weekend.
(for example, augmented gano legs in the above 2 statements)
It appears Jagex is a little reluctant to clarify lately, & I wanted to know if you believe the above still to be correct, or if it has/will be changed.