You're welcome! It also gave me a reason to review and update some of the information on that download page . :)
UPDATE (4/12/20):
Another way to do this is to use Flush Right with Dot Leaders -- but temporarily changing the "dots" to underline characters (without spaces). The result is a continuous underline up to the right margin that expands and contracts as needed.
This requires a somewhat unintuitive setting for the current document, typically made at the top of that document so that it will affect the following material in that doc.
Here's how:
[1] Place the cursor at the very top of the document's text editing area.
[2] Click Format, Line, Tab set. This opens the Tab Set dialog. (Yes, that's correct!)
[3] Type an underscore character (_) in the Dot leader character box (i.e., replace the existing "dot" character there).
[4] Type a "0" (zero) in the Character spacing box (i.e., replacing the "1" there).
[5] Click Set, then Close.
This inserts a new [Dot Lead Char] code in the document at the cursor location. (Delete it if you want to restore any of the default "dots" in dot leaders further down in that document.)
[6] Use Format, Line, Flush Right with Dot Leaders at the end of any word or short phrase and you should see an "underline" appear up to the right margin. (This method will also affect any subsequent "dot leader" feature, such as hard tabs with dot leaders and centered text with dot leaders, in that document.)
Done!
As when using the normal Flush Right with Dot Leaders, adding or deleting text on that line (to the left of the dot leaders) will adjust the length of the underline automatically.