It turns out that the dripping brake fluid wasn't the reservoir cap.
It might be the hose bwtween the reservoir and the cylinder.

At one point, I decided to replace the Brembo radial master cylinder
entirely with a spare Kawasaki Nissin one, in case it was the cylinder
itself, though evidence says otherwise. The spare lacks a handle bar
clamp, but I thought: "not a problem. I have a clutch lever with a
spare handle bar clamp". Turns out that KHI clutch ones and Nissin
master cylinder ones are very slightly different, so they're not
compatible.

Looking at the weep again, it seemed that it was actually the hose or
the connection between the reservoir and master cylinder that was the
culprit. So I replaced the hose. Turns out the hose I replaced it with
was rotten and leaked even worse. More paint lost. Old hose trimmed
and refitted.

So far, it isn't leaking, but I thought I had fixed it before. It's
not a huge problem in practical terms: the brakes are firm and hold
pressure, but this constant trivial weep is seriously annoying.

--

-Pip