Part of the workaround technique. Framing is done from real buffer very slow and focusing can be done while holding halfshutter while recording. Then back into framing again. In and out. I can work with that and capture pretty much anything I want.
I tried 2019Oct15.5D3113 and 2019Oct15.5D3113. This feature is very very useful but sometimes(mostly) does not work for me. When it works and after pressing half shutter, it backs streched view again.
When I run your Lua script, everythings are fine and recording is ok. Low fps and laggy preview but at least not streched, framing is good. (I can get 5424x2270 24fps 10bit/12bit continuous)

When pressing half shutter for focusing:

And it stucks there. I can't see correct framed version again. Sometimes works 1-2 times, then streched view again. Also freezing camera when change aspect ratio on crop_rec menu.
Am I doing something wrong?
Thanks.