[joeby]

That was a good lead, but I downloaded the plugin, and although it shows up (unlike the display-name plugin) activating it has no impact on old registrations.

I also tried to register a new account “Test User” — the confirmation screen says, “Your registration as TestUser was successful.” No space.

Although in a wacky twist of fate, the registration did also carry through to WP. And assigned the correct role once the user logged in, albeit with a changed username as indicated above.