Sorry for the late response. 

Would it be reasonable to have the syntax be something like:

| given=Jonathan
| family=Lang
| middle=LeRoy
| surname=Mr.
| {{{given}}} {{{middle}}} {{{family}}}

| given=Ranma
| family=Saotome
| {{{family}}} {{{given}}}

This way, the subobject's formatting could be determined on a case-by-case basis, and in a manner that people are used to (i.e., value on the left; display on the right).  In effect, the unnamed entry is assumed to contain an "inline template" for display purposes.  This saves you the trouble of writing a new template every time you want to format a subobject differently.

If no inline template is given, there should be a default inline template on the property page; possibly use the property page itself as the default template, and rely on the usual tricks used by template pages to define text that should only be available when viewing the property page and text that should only be available when making use of it as a template.

BTW: "subobject" is technically accurate, but needlessly long.  I'd recommend going with "object" instead.  Granted, this is a nitpick; but there you are. 

Jonathan "Dataweaver" Lang