Well... I've been digging (and digging and digging) through threads and wikis and I still haven't found anything close to what I'm looking for.

When I search for systemui.xml syntax, I find threads on adding a reboot menu option to the power button menu (by simply uncommenting what's already in the XML).
When I search for d-bus documentation, I find lots on using it for triggers, some on issuing commands via shell command-line, but nothing relevant to the syntax used in systemui.xml.

Does anyone know of documentation on the syntax used in systemui.xml?
I've tried variations on what I posted above... all show a button, none work.