UPDATE: 5-5-07
Well, I finally got a new 2-3 shifter fork (what a beauty it is!) and installed it. 2nd gear is only moderately better, but it still tries to kick out and catches when I go to shift into 3rd. Something's not meshing right, and when that happens I hear a clattering of gear teeth as if they're not quite lining up :-(
If I step on the clutch again when the gear jams, it frees up and I can shift easily into 3rd.
I wish I knew more about the inner mechanics of the tranny so I'd be better able to pinpoint the problem. Gears R, 1, and 3 work absolutely fine. And given the limited range of gear movement allowed by the shifter forks, I'm inclined to believe the problem lies somewhere in the clutch-to-2nd-gear lineup. So today I think I'm going to play with the clutch pedal travel and see if that has any effect.
Anyone have any other suggestions?