Steve,
What you described sounds like what we call a leach line.

I have one in my Taipan sail. It can run down the batten ends but these days is made into the leach tabbing on the sail.

It was a bit of mucking around to put in after the sail was made but it went in anyway. I actually used fine brass tubing
around 15- 20mm long (2-3mm dia) with the ends flaired to act as a low friction quide for the 1 or 2mm spectra line past the batten pocket eyelets.
There is a small cleat on the boom th allows it to be adjusted or just let off altogether and if cleated it will increase camber in the sail which will vary with mainsheet tension.

In the Paper Tiger class it is used quite a bit. I have even seen Tigers set up so when the downhaul is let off for downwind it automatically tensions the leach line.

In many ways it is just another toy.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!