John Mitchell's approach (a job that periodically checks for new database mirrors) would be the way I would go if automatically changing that setting really were necessary, perhaps because of consistent network blips or some such thing.

As an aside, I think your posted error message is from a previous try at creating the trigger, since that error mentions msdb.sys.database_mirroring, while the script only correctly refers to master.sys.database_mirroring. 🙂