So, it didn't work (or I did it wrong). My transmission user in the jail has ID 40001 (per the /etc/group file inside the jail). I added user 40001 to the my media_access group OUTSIDE the jail (in /etc/group in the FreeNAS install), and for good measure (after this on it's own did not work) added the transmission group in the group file as well with the same UID/GID.
I still get permission deined (file exists) issues with transmission in this configuration. Restarting the transmission service and the jail make a difference. Restarting the whole FreeNAS server removed my changes from the FreeNAS /etc/group file (I guess this is generated on boot).
So, no worky.
Did I do something wrong or is this just a no-go for some other reason?
Edit: I repeated the process, except using the FreeNAS webgui to add a user (transmission) with the same UID/GID as the jail version (4o001). I then added this user to my media_access group. No go. (Though the user/group did persist through reboot when added via this method...expected behavior).