draw.io for Confluence Server version 6.5.0 introduces the concept of custom templates. This enables you to define set(s) of custom templates that appear in the new diagram splash screen.
To configure, go to the draw.io add-on management screen at BASE_URL/admin/drawioConfig.action and select "Add-on configuration":
The "Custom templates configuration page" holds the URL to a Confluence page (of your choosing) where you define new splash screen sections as child pages. The content of the page linked is not used in the splash, and the blueprint for that page, known as the "root custom template page", only contains this help text.
Each child page of the root custom template page is displayed as a section in the draw.io new diagram splash screen (unless a child page holds no diagrams). When you have created the child pages, diagrams in those pages will show under the "Custom Templates" section in the splash:
In this case there are three child pages. To add diagram templates to these section, add the diagrams to the child pages as you would add diagrams to a normal Confluence page:
The titles of the sections match those of the child page titles and the diagram titles in the splash will use the names of the diagrams in the pages.
When a template is used, a copy of that template is saved locally to the page, the original template is unchanged.