Thanks for this.
The category would only be visible to members of the group so there would be a chicken and egg scenario here!
I’m fairly accustomed to PHP so think I could write a script that takes the email addresses from Mailman 2 (it has a Python script for that) and checks them against the Discourse user list, adding or removing Discourse users from the relevant group as appropriate. I know you said this is “probably the worst solution”…
I’ve not even set up the mailing list mirror yet, so these are just ideas so far.