Can't authenticate using SSH key when running script via launchd
So this is what I ended up doing to get it working. For my use case it is sufficient, but I don''t think it's the best solution. Hopefully someone (see @Allan's comments in OP) can come up with a more universal solution.

I added the -e option to my rsync statement which allows you to specify the exact ssh command used.

rsync -auvzP  -e "ssh -i ~/.ssh/id_rsa_scripts" '/Volumes/Norman Data/me/Documents' me@example.com:backup/


The important part in there is ~/.ssh/id_rsa_scripts - I generated a secondary set of keys for that server, with this set having no passphrase. This is different from how I connect to that box via SSH by default and thus why it needed to be specified in the rsync command.

That was the only way I could get it to work when running unattended (as a Global Daemon). Although it is ok under my set of circumstances, a passphrase-less key is not the best idea in many situations, which is why this is not the ** best ** solution. 

Hopefully someone can figure out a way to allow launchd to execute a script requiring SSH authentication using keys with a passphrase.