Can an overline be extended to the right margin without placing the tab mark there?
What are you trying to do?
And what's the issue with adding another tab?
I can immediately think of at least two other ways to do it or fake it, such as Frame Above Pgf and single-row Table that is mostly borderless.
Is this a line from the Reference pages? make sure it's actually as long as you need it to be; it can be longer than the containing frame on the Reference page.