No, WordPad does not seem to respect paragraph borders either. I am sending something like “{\pard \brdrb \brdrs \brdrw10 \brsp20 \par}” but that does not show any line at all.
I can create a table cell and underline that one (that works) but I do not want to do this as I do not know the required cell width from the application. So I was hoping to just underline the paragraph and sent a bunch of “\par” to simulate fill-in lines on the printed output