You can number lines in Writer, and you can define reference marks, but you cannot refer to a line number (only to page number or reference text, etc.).
Therefore I suggest to allow referring to the line number of a text reference.
This sounds like an awesome enhancement request - it would have to be flushed out to figure out exactly how we would do this but ultimately I think it's a pretty sweet idea. Putting to UX to see if they can flush out the details. Feel free to comment and provide additional suggestions as to how this would work. Thanks!