So, having been rained out of sailing over the weekend, I decided to try again today to figure everything out. Following an epiphany of sorts, I swapped out the square lashing on the boom (holding the boom close to the mast) for a "Bleater" (GIS nomenclature for a line to keep the sail bundle from moving foreward as the sail is raised). Using the square lashing this spring resulted from "faulty" muscle (brain) memory from rigging my GIS and short-term memory loss (I've used a Bleater on Gardens the last two summers).
The Bleater loops around the boom, then around the mast, and forward to attach near the end of the boom. A little trial and error to get the Bleater length correct (to position the tack of the sail 400mm ahead of the mast), and the Bleater keeps the sail bundle from moving forward as the sail is raised.
How does this help the lazy-jacks hold the sail bundle horizontal? As best as I can figure, when using the square lashing allows the bundle to move foreward when the sail is raised or lowered. That movement changes the geometry (and pivot point) of the lazy-jacks, and the foreward end of the boom drops while the aft end raises.
Whether that is really what is happening or not, the result is that with the Bleater the bundle does not more foreward and I can set the lazy-jacks to hold the sail bundle horizontal at whatever height I choose (well, with reason).
One solution I considered was parrel beads. I didn't have any beads to try, but it seems to me, the Bleater is a variation on the theme of parrel beads.
Now, to sort out reefing...