EOS R5 II: Why can't you set a custom button for Subject Detection ON

[Kwillo]

Hi Team,
Recently purchased the R5ii and already own a C70 primarily shooting video. 
I've been trying to mirror as many settings on the R5 to match how I shoot on the C70 and one thing I cannot make work is setting a custom button to turn Subject to detect ON for video. I'm looking at setting it to the front DOF preview button. 
The closest I can do is set it to 'Eye Detection' but this requires 'Subject to detect' to already be on.
The reason I'm looking to do this is when I'm filming people they are often showing things to the camera with their hands, I don't want subject to detect on because it will just stay locked onto their face, I want to be able to easily switch that on and off.
Surely this is possible? It appears I can change so many other functions hopefully I'm just missing something easy. 
Cheers, 

[shadowsports]

Greetings,

I've read posts from others who have expressed interest in button customization for pre-capture and subject detection.  I would suggest leaving product feedback for Canon..  You can do this on the Canon USA homepage.  Look in the upper right corner for [+]Feedback, select product and leave your feature request(s).

[Waddizzle]

I recommend exploring a custom shooting mode.  If you could setup a button to alter the setting, it would most likely require you to press a button and keep it pressed, like how BBF works. I would be surprised if you could toggle it on and off.  

Changing shooting modes on the fly isn’t the best option for action photography.  I think you may discover not using the most advanced AF features will yield excellent results, too, especially shooting video.   

[Kwillo]

Thanks shadowsports. I'll add it. Also seems strange you can set a custom button for Pre Record in video but not pre capture in photo. 

[Kwillo]

The custom modes are already taken up (different frame rates and formats) and not suitable for changing mid shot. Currently the BBF button can be toggled for some settings such as Eye detection.
In this instance I'm not shooting action photography, it's just filming a show and tell. 
It's essentially just a way to force the camera to search for a face to focus on mid shot without touching the screen.
On the C70 I have the assignable front button 11 set to 'Face det. & Tracking' and it works amazing.