Did work perfectly - until now, when I upgraded to Thunderbird 13.0.1
EDIT: Great! The update fixes the problem.
Reaction of developer from bugreport to fix: under 24h, that's fantastic! Thanks for the good work.
I've posted an update to this at https://addons.mozilla.org/en-US/thunderbird/addon/new-account-types/versions/?page=1#version-14.1 - but standard AMO policy is to disable new binary addons until they have been reviewed, so this might not work until after the review.