Hi Stuart,
I concur with your statement:
“... 'original Lucas' rated the RM21 for 10.5A @ 5,000 rpm and, as I say, the RM23 for 14.5A @ 5,000 rpm; 75% is 7.875 Amps and 10.875 Amps respectively, an extra 3A at low rpm is a bit more than "differ very slightly" on these old heaps.”

You are however, placing much faith in the advertised figures. I agree that is all we have to go on, till we know better.
Promotional ads from Lucas in ~1980 did suggest that the 3-phase system would solve all my issues (which it did).

My situation was a pre-unit with failed magneto. Mistral mag replacement system with Rita sorted that for 30 years. But the new installed RM21 I found inadequate for the demands. So I went to the high output RM24 with 2 Zeners kit from Mistral. I ran this for several years with the Rita and no battery with no problem (no capacitor). It went to Morocco and back like that in 1984.

Since then I have run a battery, and have changed to Boyer IV. It works well (Boyer III it worked well too).

Never known of these low voltage issues, but I also make high quality bespoke harnesses, as yourself.