The RTE field in Sitecore is a little bit old fashioned and a little stricter than the most of us would like, especially when you are working with tech savvy editors who may prefer to work directly in the HTML. The current project I am working on is based on the Foundation framework and like most frameworks a lot of the features are enabled using data-* attributes.
Unfortunately the Telerik controls used in Sitecore try to be super helpful and strip out any invalid XHTML. Also unfortunately data-* attributes are HTML5, and not XHTML 😦
tl;dr Adding custom drop lists in RTE is already built in to Sitecore
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.