Unfortunately the Silverlight TextBox does have a very limited API and you cannot retrieve the text offset index based on the mouse position.
I believe that there may be a way to estimate the current row in the TextBox if the text in the text box is not wrapped. You can inherit the text box and get its scroll viewer. Then you can use its ExtentHeight to get the total height of the text and then do some calculation to find the text row height and use the mouse position to display an overlay marker and then insert something there. This though sounds like a hacky solution and I cannot think of a workaround if the text is wrapped.
the Telerik team