While I can understand the need for it, I am struggling to think about how it could be done in casparcg properly.
Caspar is all about a channel having multiple layers playing. So while it is unlikely that anyone wanting captions will try to play multiple things on a channel, someone will. What should happen when it does. Should it take the captions from the lowest layer? or uppermost? can they be combined? Does it matter if one of those clips has been muted?
From what I have read from github, it might be that the decklink needs to be fed yuv frames to be able to add captions. There isnt a flow in place to get yuv frames, only rgba. So itll need a new flow to convert frames, which may become a performance drain (when enabled). Also consider that the only way to feed a decklink frames with alpha is in rgba. So if you want an alpha channel out and to be in yuv mode then we need to feed two decklink outputs perfectly synchronised frames ourselves, which is also something new.
So it sounds to me like there are quite a lot of areas inside of casparcg that need changes to handle captions. But it is doable, and if someone wants to do it I am available to discuss plans and to help them get it merged.