I found the source code but no answer. I ended up editing the source code, and while that works, I would much rather do something else than that if possible.
To have a more elaborate answer to the rest of my question, basically it's not something I think is feasible to directly edit the video file WHILE it's playing. I also don't know if it would be a good idea to save every single and just combine them. I was able to find an extremely efficient, but niche solution by modifying the preview frame while it plays, and having that persist across every new frame. Then I saved JUST the overlay to a file, and can use that however else I feel.
I have seen no other threads/users actually deal with moviepy in this way, so feel free to PM me or ask on the thread if you want more info.
Source code here