
I rode around with it for awhile, and played with the reach (fore-and-aft) and height of the shift tab. I discovered that the Touratech shift tab sometime pivoted on its spring, and muffed downshifts. Also, the tab wasn’t quite long enough, and its rectangular hard-edged end felt unsmooth to my foot.
These problems are addressed in the final version:

The shift tab is now made up of aluminum spacers bolted directly to the tab end of the shift lever, the same type as the spacer between the two pieces of the shift lever itself in the prior version. Three spacers were too long, and two were too short - I cut one with a hacksaw, and found that 2.5 spacers was ideal. The piece that holds the shift tab (which replaces the earlier "tab end") was cut from a steel L-bracket barely wider than the spacers.
My friend has a lathe, and he chamfered the edge of the end spacer, and countersunk the end. The socket-head cap screw protrudes just a bit (maybe 1 mm) from the spacer; with the rounded spacer edge, the end of the shift tab looks and feels elegant. I don’t think I’ll paint it.