It's possible to define your own custom templates for normal project types, such as templates for C# files, for a Class Library project.
What about a 'Database Project' scenario?
I would like to define a standard template for adding a stored procedure, which uses the company's conventions for all stored procedures, such as standard comments at the top.
After looking through an MSDN article on the topic, it seems as though the Database Project isn't supported:
Figure 5 TemplateData Elements for an Item
ProjectType | Indicates the type of project that this item can be added to. Possible values include CSharp, VisualBasic, VisualC, JSharp, and Web.