Nice one. legacy RAW still alive .
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Idea. How about disabling H.264 proxy once RAW starts to record in mv1080 to maybe fool eosm to stay in mv1080 mode?
Not sure how you would do that, other than stopping H.264 recording (which will bring you back to 720p on EOS M).
The two recorders are pretty much independent (Canon's H.264 has no idea somebody else is also recording, and ML's raw recording doesn't care what Canon's H.264 does - it simply starts and stops H.264). They both get their image data from LiveView, so they will be in sync (without doing anything special).
The start and end moments will differ though (H.264 is started first and stopped last).
Bonus - you get free sound, without any coding effort from our side.