A thought: Fabricate arms says "Treat this as a spell of the same level as the expended spell slot." With that in mind, could one double the duration of a fabricated weapon or armor with the use of the extended spell magic hack?
My only devil's advocate counter argument would be that Extended Spell's text says "When you cast a spell...", but does "Treat this as a spell..." mean you're considered to have "cast" it.
When casting a spell, you can apply no more than one magic hack that affects the attributes of a spell
And completely changing a spell's effects to give up whatever it should do to conjure equipment instead is definitely affecting the attributes of a spell. So no, by RAW.
I disagree. Fabricate Arms is cast as if it were, itself, a spell of the level of the slot consumed. It is NOT modifying the attributes of an existing spell. You could absolutely use them together according to RAW.
These two magic hacks do not work together. Fabricate Arms is a (Su), Supernatural Ability. You activate it, you don't cast it.