I recognize that this is a 3 month old thread but in case you are still interested, i've had limited success.
Like th OP I am seeking to overlay video with motion text over an existing video capture source (webcam).
Stumbling blindly through the tools I prepared my video with Blackmagic Fusion. I animated my text and specified a transparent background on the text tool. Adding the "saver" tool I set the format as "PNG" (because it supports transparency), i checked the option "attempt to save Alpha". I then named the file with an MOV extant producing th MOV file that I specified in the scene in OBS (not MP).
Yhe video partially plays over the camera transparently but it does not play fully. It poops out a few seconds into the video.
I will continue to try and learn enough to refine my results.
I will also look forward to hearing any suggestions or corrections that more informed users might offer.