How hard would it be to write a lightweight <subpage> tag that scans its contents for properties, extracts them, and appends a {{#subobject}} built out of the properties that it found?  For example:

<subpage name="Forrest Gump">
[[given::Forrest]] [[surname::Gump]] is the [[title::Shrimp Man]] of [[org::Bubba Gump Shrimp Co.]]
</subpage>

would become something like:

Forrest Gump is the [[Shrimp Man]] of [[Bubba Gump Shrimp Co.]]
{{#subobject:Forrest Gump|given=Forrest|surname=Gump|title=Shrimp Man|org=Bubba Gump Shrimp Co.}}

I suspect that <subpage> would be at least as popular as {{#subobject}} as a way of adding SIO-like semantics to a page. 

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Jonathan "Dataweaver" Lang