I did some further research on this line. It's called a "luff line" (imagine that!), and it's purpose is to make reefing easier.
Without it, it is difficult to get the reefing tack down to the reefing hook, as all the slugs below the reef point pile up
in the sail track and hold the reefing tack up too high. In some cases, you would actually have to remove the slugs
from the track in order to reef! With the P323's "track-gate" system, it's not something I want to do on a pitching
deck with a storm bearing down!
(I know, I know! I should have been reefed BEFORE that point! )
The luff line also a convenient place grab to haul the sail down to
the reef point, as there's usually some side-loading while reefing, and the slugs tend to bind in the track.
I've found recommendations that it should be tied off either to the gooseneck, or to the tack of the sail.