UPDATE: found UK maker of merino cycling wear! Including jackets!
I'm about to pull the trigger on this:
http://www.embersmerino.com/merino-c...acket_p104.htm
I just stumbled across Embers Merino (based in Derbyshire, England) and found it seems to be highly regarded by members of the YACF forum - which has a lot of long-distance/audax riders. So fingers crossed the quality is good. Whether or not it addresses the specific needs I have is another question. I'll post my thoughts once I've got it and have worn it for a bit.