Updating to iOS 14.6 RC 2 appears to solve the problem. Which is good, because I had run out of chicken entrails to read, and the usual “Eye of newt and toe of frog, Wool of bat and [here the forum software redacted a perfectly innocuous word] of dog…” potions just weren’t working for me... ;-)

Update a day later: current builds are now working in 14.5 (perhaps irrelevant now that 14.6 has been released, but interesting nonetheless). Is it possible that the multicast entitlement took a couple of days to "burn in"? Quinn mentions "wait a day, and try again" for the entitlement not being available for an Apple ID; I wonder if a similar delay occurred for the entitlement not working within the app, which a rebuild a day later fixed.