Interesting. My Office\14.0 and \15.0 don't even have a Spotlight leaf by default. Some questions that come to mind:
- Also, I wonder what you do if you want more than one template? Or is it for a category that maps to a directory full of templates?
- What are the rights of an install program to create these registry keys?
- Does the solution publishing tool create these registry entries and xml files?
I'm investigating some more...
I have also investigated a bit

- By default, there is no "Spotlight" key, one has to create it manually.
- It looks like you can have as many templates and categories as you want. To create a category, you register a new file under [category name] in the registry. To add more templates to the category, you append them to the list in the XML file - seems to work fine.
- The key can be created by a restricted user (it's under HKCU). No Admin permissions required.
- The "solution publishing tool" does something completely different (it uses the "PublishComponent" MSI table to fill the publication data windows installer registry). Nothing like "Sportlight" or this XML file is created.
That means, it looks like Visio has 2 alternative (hopefully not conflicting) ways to fill templates on the start screen. First is the "old" method which gets the data from the windows installer; the second one is this "Spotlight" way which obviously comes from the Office package.