The issue with the bdstemplate above is the Creator tag. After lots of futzing and a reread of the embarcadero page I discovered that there only 2 acceptable values for it.
Project Type Item Creator Attribute Value
C++ Projects Creator="CBuilderProjectRepositoryCreator"
Delphi Projects Creator="DelphiProjectRepositoryCreator"
Then I discovered the category tag within the RADStudioRepository.xml file which is in your %AppData% folder. Adding this tag as a part of the item causes it to appear in its own folder under "New Items".
<Categories>
<Category Value="Borland.CBuilder" Parent="Borland.CBuilder">Nutana Projects</Category>
</Categories>
which is very cool. I briefly got the "Nutana Projects" folder inside the C++Builder Projects folder but I don't know how I did it and can't repeat it.
When creating a new project this way it asks for a destination folder then throws an error that it can't open .cbproj. Everything is ok though, you just have to open your new project manually. I suspect it has something to do with the FilePath or DefaultProjectName tags, which don't seem to be doing anything.
And there doesn't seem to be a problem with the external dependencies, although I haven't tried moving the project somewhere odd, to see if it adjusts the paths.