If you are operating in no-fill mode, why not use .tc to define the
tab character? You can then set up the left-right justification and
troff/groff replaces a tab character between left-hand and right-hand
text with a multiple series of the character defined by .tc. You
have to use .ta <dimension> R to get the right-aligned segment where
<dimesion> identifies the location of the right-hand margin and R
indicates right-justified text to that margin.
This is commonly used when constructing a table of contents or index
page. I think it's called a "leader" (fading memory :-) ).