If anybody is interested, I've got custom settlements to work on my mac by using a wineskin wrapper intended for another game.
Technically it's using the windows version of m2, not feral's port, but it seems as close as we'll get to a fix. This should also work for MOS, DaC and any other submods people have been struggling with.
Download link is here: http://portingteam.com/files/file/80...ire-rebellion/
When you run the wrapper, log into steam, redownload med 2 and copy/paste your third age folder into the wrapper by right clicking on the wrapper itself and clicking show package contents then finding the med 2 mods folder there.
Since this is the windows version, you need to rename the third age folder to one of the kingdoms expansions then select that to make it work. I rename 'british-isles' to 'british isles original' and 'third_age_3' to 'british-isles'. To play TATW, i simply select the kingdoms Britain campaign.
EDIT: Just to confirm I have tested MOS using this method and it works.