Well, I received the 2BA screws yesterday, and they're no-go, just like the #10-32 screws I've also tried -- they'll both look like they'll go in, but they stop after about 3/4 turn. Because it seemed to me, on closer inspection, that the internal thread pitch of the braze-ons is larger than that of the screws, I sanded down a bamboo chopstick so it was just a bit larger than the major diameter of the screw, and force-threaded it into the braze-on. Imagine my surprise when I found that the internal thread has about a 1 mm pitch! I can't seem to find any sort of screw having that major diameter (about 4.7) and that coarse a pitch, unless it's a UNC 10-24, or a Whitworth BSW (coarse) 3/16-24, but I'm not inclined to believe, unless the bamboo stick slipped under threading (doesn't look like it), that the threads could be 24 TPI. Still, I will find a 10-24 screw and see if that fits any better.
The British Cycling standard (BSC or CEI) is a good guess, but I can't find any evidence that 3/16-26 was ever "a thing", which would be the only "fit" for this thread.
The bad news is that rethreading to the standard used by Campagnolo-style braze-ons (5 x 0.8 mm) probably won't work, since the thread pitch of the Cyclo is not close enough, and the 5 mm (nominal, most that I've measured are actually smaller) screw diameter is too close for any thread without a similar pitch. The only "retap" that I can see working is 6 x 1.0 mm, which would mean drilling out any shifter I wanted to use on it.