I've used my own crop_rec presets on vacation for some testing.
It works, but once in a while the first 2 frames of a mlv file are corrupt (purple, broken etc). But the rest of the mlv is fine, so it is usable.
Also when this happens, console shows up giving a error message (probably because of the weird file size of the few corrupted frames at the start).
This is what I have for now:

You must be in the correct video mode to use the presets, use zoom button a few times to scroll to 5x zoom/ 10x zoom etc, to be sure all registers are altered.
When used in the proper video mode, shutter times are accurate and can even be expanded to 1/25000th of a second, depending on the crop_rec preset.
To get this full range shutter time option, use q button in ML menu on the crop_rec option, you get a sub-menu with 2 shutter range options, 'original' or 'full range'.
-1832x800 50fps 720p mode : This is for use in Canon 50 fps 720p mode, doesn't work in Canon 1080p modes, not for use in 5x zoom mode, but for normal standard view.
-1832x1230 35fps 1080p mode: This is for use in standard Canon 1080p, not for use in 5x zoom mode, but for normal standard view.
-2688x1166 25 fps liveview: This is for use in standard Canon 1080p, and must be used in 5 x zoom mode. This mode has working liveview, be it a centered crop of the actual framing. Use ml preview to see the actual framing (depending on your settings, this happens automatic, otherwise use half shutter press to see preview)
-2880x1200 25fps ML preview:This is for use in standard Canon 1080p, and must be used in 5 x zoom mode. liveview is scrambled/broken, use ML preview option to see framing (see ML raw video menu for preview options)
-3840x2160 10fps ML preview:This is for use in standard Canon 1080p, and must be used in 5 x zoom mode. liveview is scrambled/broken, use ML preview option to see framing (see ML raw video menu for preview options)
-5120x2142 8.3fps ML preview:This is for use in standard Canon 1080p, and must be used in 5 x zoom mode. liveview is scrambled/broken, use ML preview option to see framing (see ML raw video menu for preview options)
-5472x3604 4.8fps ML preview:This is for use in standard Canon 1080p, and must be used in 5 x zoom mode. liveview is scrambled/broken, use ML preview option to see framing (see ML raw video menu for preview options)
This last one can be used as a sort of electronic shutter photo mode, with the so called full shutter range option, you can use fast shutter times, be it with a little rolling shutter effect. The fun thing is, the writing speed with the 6d is about just enough to get 1 or 2 frames in this mode, before the camerabuffer is full and stops.

So it's a perfect silent electronic shutter photo mode
I used these preset for a while and I like the 2688x 1166 25fps with liveview the most, and I think it is the most usable one, because it has working liveview.
Also writing speed of the 6d is not sufficient enough to get long recording times in 2880x1200 for example.
Most of the times I even lowered the resolution to about 2560 x 1072 to get longer recording times, or 2400x1004 resolution.
For now these crop_rec modes only work with the 14 bit lossless recording option, 12 bit lossless and lower don't work, because liveview resolutions are bigger then standard.
And finally, here is the crop_rec module file for download, put in on your SD card in the ML/MODULES folder, this module should work with the Crop_rec_4K build for 6d on the experimental builds download page.
https://drive.google.com/open?id=1-X2OB14zsZBKdvbJSQC6LokNOgMLXVQvEDIT: these crop presets don't work that will with FPS override function. FPS override can be activated, but it could be that it destroys the usability of the resolution, so (random) corrupted frames.
FPS are sort of baked in, in these crop presets.
If people are looking for 24 fps crop presets, I can make a special 24fps crop_rec build with the above presets.