Thanks for the feedback.
The conversation view uses RadJumpList internally so we'll expose the empty content related properties in the next release.
For now you can simply Inherit from RadConversationView. Then, override the OnApplyTemplate method and call this.GetTemplatePart<RadDataBoundListBox>("PART_ListBox"); and use the result to tweak the empty content.
Please write again if you have more feedback, we appreciate it.
the Telerik team