tl;dr Adding custom drop lists in RTE is already built in to Sitecore
A1 on StackOverflow asking if it was possible to add custom dropdown lists similar to Symbols caught my interest. I’ve looked at the RTE implementation before but not in any depth since the Telerik RadEditor control was updated in Sitecore 6.5. I’d also added some implementation details to a previous question.
Whilst looking through SetupToolbar in Sitecore.Shell.Controls.RichTextEditor.EditorConfiguration it turns out that support for custom drop lists is built in to Sitecore already so it is fairly easy to add.