Actually I just double checked and I was incorrect. I just remembered that I have another app on my phone called AdBlock[1] which is responsible for blocking requests at the network layer. They run their own DNS server and create a custom list to black hole network requests that match certain formats. If you add Facebook as a custom rule to AdBlock, that will prevent Messages from pre-rendering content and also block any messages to facebook from any service on your phone as long as you're connected to their VPN.

Sorry about the confusion... I'm really doing a lot to keep myself off of Facebooks radar.

- [1] https://itunes.apple.com/us/app/adblock/id691121579?mt=8