Kinda old topic but those who have this problem. I was able to resolve it like this:
1. Installed plug-in and stopped
2. Add mount points to jail
3. entered shell through jail
4. followed step 2 but instead of /var/db/PLUGIN which i cant seem to find is used /var/run/transmission (idk but it worked)
In the jail, add a user with a matching UID as the owner of the data dataset/folders.