-- Your group requires hashtags,
-- You only have one available, and
-- It locks the topic.
I know it's a kludge, and will require some training of your subscribers. But it should work.
Bruce's idea above may not be that much of a "kludge" after all. You could include the required hashtag in the "Subject Tag" under "Message Format" (see settings) which will automatically include it in every message. (Note I have not tested this, but we add text there which is helpful. The only problem would be is if for some reason it strips hashtags from the "Subject Tag.")