I've managed to confirm the issue. I've logged it under:
Bug 10319: WebUI can't record text input into ComponentOne's Silverlight RichTextBox
We'll look into it but we can't guarantee we'll be able to provide you with a working solution quickly. Telerik also provides Silverlight Controls. While we guarantee 100% flawless integration with Telerik's controls - we cannot promise the same for ComponentOne's. As you can see WebUI Test Studio works correctly with Telerik's RichTextBox:
Maybe you can come up with some sort of workaround based on Pasting (recording CTRL+V) until our engineers look into it?
Let me know if there's anything else I can do to help you!
the Telerik team