Got something to play with.
New crop_rec build for 6d, with Custom Resolution preset, now you can dial in any resolution you like (The preset configures the corresponding max fps for that resolution).
The preset is made for use in 5x zoom and is 1:1 pixel readout.
Select the "Custom Resolution Experimental" crop preset from crop-rec menu.
After you selected the preset, you can enter crop_rec submenu with the [Q] button on your camera:
In here you can select Horizontal resolution (Goes from 1656 to 5472) and vertical resolution (goes from 640 to 3648)
Dial in any resolution you like and it probably works
Could be that there is a slightly difference between the resolution dialed in and the one selectable in RAW video menu (horizontal resolution can be 8 pixels difference sometimes, and depending on the aspect ratio chosen, you may not see the full vertical video resolution, in that case choose another aspect ratio, like 1:2 or 1:1)
As you can see in the screenshot above, there is also a safetymargin setting, this is the overhead on the B-timer(coupled to vertical resolution).
A safety margin value of 64, seemed really robust to me. But in case you get some corrupted frames, you might want to try to increase this value.
The safety margin is directly coupled to B-timer, so it can be used to finetune fps. But if it is set to low, you might end up with corrupted frames.
Trial and error.
If your settings are too critical and you get corrupted frames (could just be only one corrupted frame at the beginning on your video file), you may start to see scrolling text on your display.
You can get rid of this text box on your display by going to debug menu and highlight 'show console' and press the 'set' button on your camera twice (turning camera off and on also helps, but the other action is slightly less annoying):
Now finally, here is the link to the module file:
https://drive.google.com/open?id=1mJkTJlPMmckeyM84f06jCLQ2IPM1yU6YUpdated version, with Canon liveview (Canon liveview works between Horizontal Resolutions range (1840 - 2880) and vertical resolutions range (908 - 3648)For anyone interested, the source file:
https://drive.google.com/open?id=1aBmQWSX6fDQRdtZwK-mApYjDPlEJyNBhUpdated version, with Canon liveview (Canon liveview works between Horizontal Resolutions range (1840 - 2880) and vertical resolutions range (908 - 3648)Good to know, this preset is experimental, tested it myself, did get some camera lockups when accidentally recording in 1x zoom mode (error 70).
But haven't seen anything more worse then camera lockup (mirror suddenly goes down, and camera doesn't respond), which is fixed by pulling out the battery.
Re inserting the battery and turn on the camera again.
But With the safety margin of 64 (default setting) I think it is rather stable.
Now you might question and think about doing what all idiots would do with this tool...'How fast is the fps when the resolution is set to lowest possible settings'.
Well I can tell you that when 1656x640 is chosen as resolution, you get about 67fps.
Seems stable to me in 5xzoom (got lockup when accidentally recording in 1xzoom 
)High resolution settings with slow fps, might give slow response in giving you ML preview. Could also be the case with low resolution settings with high fps.
In that case, just wait a few seconds, if that doesn't help, you might want to press the 'menu' button on your camera twice (this makes sure all registers are set to chosen value)
Real use for this preset could be finding a desired resolution for 30fps. In that case, you have to choose a resolution and see in ML preview how much fps that resolution is.
If you need more fps, you need to lower vertical or horizontal resolution.
When you have the desired fps, for instance 30 fps with a certain resolution, you can finetune your recording resolurtion in the Raw video menu. (by choosing proper aspect ratio and choosing the desired resolution(use top scroll wheel on your camera for small increment steps (8 pixel horizontal resolution).
EDIT: Canon liveview doesn't work in this extreme preset, you can make use of the ML-preview (see Raw video menu)
Furthermore, Whatever resolution is selected, horizontally, it always should be in the middle of the sensor, couldn't get vertical resolution in the middle, I believe it starts at the bottom of the sensor and goes up when resolution is increased.
EDIT 2: playing with it right now, think it works really well with framerates up to 30 fps. If you go higher then 30fps, it’s sort of hit or miss. Trying to find decent 50fps, but small lower part of the frames are corrupten. Seen this before with 75 and 100 fps experiments. Will see if I can fix this.
1656x640 at 67 fps works like a breeze though
