Hey Colin,
Thanks for getting in touch with your question.
Please note that we are limited in supporting customizations. I can give you some pointers and ideas, but you will need to do the heavy lifting. Thanks for your understanding!
If I understand correctly then you want to pre-pend the data to the body, before it goes out from your TEC calendar to your Facebook page. And you do this with your custom plugin.
I guess with your plugin you will somehow need to access the event data, so you can feed it to Facebook.
In our documentation you should be able to find functions (e.x. tribe_get_organizer_details () ) that help you pull the needed data. Then stick it together with the event description before it goes out.
Does this help? Did I understand your question correctly?
Cheers,
Andras