i've dabbled in OBS' code and actually this should be possible. you might need to modify the "Media Source" element to add the GUI controls for FTL-specific stuff, but all the protocol support seems to already be in there and the actual media transport is RTP, which is already supported because the Media Source supports RTMP streams.

you'd have each of your different origins stream to their own private FTL paths, then in your OBS you'd create media sources from each of those paths then push your final, mixed stream to the public FTL path.