Update:
I sent Koolstop an email, and was reasonably quickly contacted by their local agent - who, once I'd repeated the question contacted the main office again. Not entirely surprising, experience with what works on plastic rims is limited. Anyhow, main office responded, through local agent, that KS Salmon was the way to go. They claim they'll give good braking w/o smearing.
But there are none available locally, so I had to order from abroad.
We'll see. I'm gonna need a replacements soon. I've done maybe 60 miles on the bike, and the old pads are shrinking at an alarming rate.