Would it be possible to add the possibility to recognize an interwiki link in MediaWiki:Gadgets-definition and load file from there?
Topic on Extension talk:Gadgets
See ongoing (but stalled) work on Extension:Gadgets/Gadgets 2.0. (phab:T31272)
Actually, global gadgets is not planed for gadgets 2.0 anymore. It is only going to happen on 3.0, if it ever does (see Extension:Gadgets/Roadmap#Gadgets_2.0, phab:T22153#2736770, phab:T31272#1469722).