You are right that RadRichTextBox does not support shapes at this point. We have this feature already logged in our backlog and you can vote and subscribe to the public item
in the feedback portal in order to receive updates on the matter.
Do not hesitate to contact us if you have any other questions.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.