Kevin,
You just answered your own question.
Use an editor widget with no scrollbars and set the height to 0.92 or 1.0 whatever, and set the maximum characters to whatever you want, say 30 or 40! Ensure you have RETURN-INSERTED set to false, WORD-WRAP set to false and LARGE set to true (that's what allows overstrike).
It's that simple, except .... if the user hits CTRL-RETURN you get a return inserted and a new line appears in the editor, effectively losing sight of the previous line ... but you can always code that one out using triggers ;¬)
Hope this helps.
Rich