I recently had the need to add a new template type for a single site on my multi-site system and wanted to be able to provide insert options for this template on content folders only in this site. The content folders are a global template and are shared across multiple sites so adding this new template type to the insert options of the content folder was not a viable option.
*Update 2
I also found that there was an issue with the <using> statements as well that required an attribute to be added. When I opened the Configure-Assign (Insert Options) or tried to add from template, I would get the following exception:
The type or namespace name 'EventArgs' could not be found.
Once I added an arbitrary attribute to the <using> tag it was fine.
*Update 1
So Sitecore Support got back to me pretty quickly and told me that if I just add an attribute that makes the reference unique that's how to solve the issue. Here's the relevant code: