Perhaps have the list owner create a temporary Yahoo Account, make it aI'm curious what makes that a high bar?
I was thinking that for most people using or creating a least-privileged mod account would be a lower bar than figuring out how to export the email address list from one's Yahoo Group. Especially for groups of over 1000 members.
The one potential "high bar" I can see is if one doesn't already have a "test" account or other fallow account to use, and also doesn't have a mobile phone number to give to Yahoo's account creation process.
Here's another idea: What if, instead of doing the transfer all at once,Advice to moderators: if you have any known malcontents in your group leave their address(es) off the list you give to Mark, lest they veto the move.