Best Way to Rig a Main Topsail

[osteoderm]

Of all the various suggestions/possible improvements, the jackline seems the most interesting. To keep down the number of jack-things, i've always called tops'l jacklines "leaders". The leader is a line fixed aloft to a point on the masthead/topmast (depending on tops'l configuration), running down the aft side of the mast, or sometimes on the quarter, and belayed on deck. A siezed-on fairlead or hank on the tops'l luff is let onto the leader, which when hauled taut at deck level, helps set up the luff closer to the masthead/topmast.
This is usually a development for free-luffed tops'ls. With a jackyard tops'l like yours, it wouldn't help much with the set of the sail, but might help with raising/lowering. Pressure from both the wind and the tops'l sheet will tend to move the top of the jackyard aft and to leeward, which in turn pushes the heel of the yard forward and to weather, effectively keeping the luff in check. The tackline keeps things in control.
Raising a jackyard tops'l, however, is more of a hassle, as the jackyard wants to fall over; to control this, you typically have to keep the tackline tensioned, easing it out as the halyard is brought in. With a leader installed, and run through a fairlead at the heel of the jackyard, you now have hands-free way to keep the heel (and thus, the long top of the jackyard) from reeling about too drunkenly as it lays aloft.
The traditional set for a tops'l is to weather of the gaff peak halyard falls, supposedly to add support to the body of the sail. Still, it's pretty obvious to most that setting to leeward makes for a superior sail shape. With a leader rigged, you could only raise or lower the sail on one side, however, once rasied you could "horse" the tack over the peak halyard falls with another tackline... Or set up two leaders on opposite quarters, and set the sail on the side that favours the longest tack of the course to be sailed... how many lines aloft do you need?
For the sake of performance and simplicity, some non-traditional jackyards are worth considering. In lieu of wood, how about aluminium or 'glass tubing? A beautiful wooden jackyard could be made of teardrop section, with an aft-facing groove to accept a boltroped tops'l luff, instead of the more common lacing.