You certainly need to overlay any graphic before it gets streamed to a CDN. I’ve used apps like Live:Air Solo (iOS) that can do simple images, text and videos and stream to any CDN.
If you really need to overlay CasparCG templates, you could set up a local rtmp server that receives each mobile stream, grab it in a CasparCG channel, and streams it again to a specific live stream key (facebook live, instagram, etc).
The thing is, you need a huge amount of upstream bandwidth and a rock solid WiFi network and still end up with some hiccups unless you set a very long cache time
Another path is to set up an Airplay or chromecast receiver that you can input to Caspar with only the camera showing up in the stream. (This reduces latency but increases instability).