Hi Daniel,
They may not exist in the current form, but a "local scope" will always exist. Template parameter names are local the the current template "call". They have to be managed somewhere, so mediawiki will always have some way to manage data attached to the "current call", and we can tie into that to put the "default item" into that "current" or "local" scope.
I am convinced if you can explain: the scope of this setting will always be identical to template parameters. For example, it will not be inherited if a template calls another template.
My own point, slightly more generic than this discussion:
Whenever you use "data item" or "item data" (both forms exist, with "item data" seemingly being an abbreviation for "data item data" ;-) ), I start to think "what was this?" - the snak, the topic, the property values?
"Topic" seems to be an excellent replacement for data item - with the added benefit of tying into topic maps.
We established the "item" terminology in the data model. The syntax spec has to be consistent with this. We could change "item" to something else, but we'd have to start in the data model.
Yes I know. I just mentioned it because I was experiencing myself being confused here -- just as an example. If you think there a chance to discuss the data model with respect to how the thing is being called, please open a discussion and I will gladly comment. I do respect your constraints in what you can discuss when.
Gregor