A few years ago, for Ygroups, one of your GroupManagersForum wrote a Chrome app that scarped entire membership lists from any group you were a member of but only gave complete email addresses from groups you owned or moderated.
This was after Yahoo Neo the Nefarious was dumped on us.
I still occasionally use the tool and it works just fine for moderators who what a complete list.
My copy is only existent in my Google Chrome account and I would not know how to extract it for others.
Possibly you had the original information and executable?
This would seem to provide the "membership only transfer" requirements. Possibly Mark could input the .CSF file of members directly into one of his magic tools?
Founder, Fuchs Friends ®
Founder & Executive Director, The Corneal Dystrophy Foundation