It is not. The line number counter is not public at the moment, and counter(par.line) is not used for anything. Sorry!
This is because the line number counter is special. It is not stepped in the document flow, but outside of it, within the page margins; there, it is stepped exclusively based on the height of the associated lines, so its value does not respect the positions of the lines in the document tree, just their height. For example, if you write LineA #place(dy: -5em)[LineB], LineB will be numbered “1.” while LineA will be numbered “2.” because LineB appeared visually above LineA. For usual counters, the opposite would happen, regardless of the visual appearance.
In addition, having, for example, 5 lines doesn’t mean you will have lines numbered from 1 to 5. If they have the same height, they will collapse into one line number.
Due to this, we cannot expose the line number counter as fetching its value or stepping it within the document flow would be undefined. So, to answer the question:
That is not easily possible at this moment, since line numbers don’t work like other counters, thus they are not exposed.