Let´s start 2019 with some eosm love. The setting is called:
4K squeeze 24fps
It´s working continuously with 10 bit and build files when set to 2.35:1 through crop_rec submenu that gives files sized to 4000x1704. The files are actually recorded into a squeezed mode(4000x696 or 4000x 568 when 2.35:1 is selected). Thanks to mlv metadata the upscaling is done with the default scale tag so it´s merely a question of opening up the file in Mlv App or acr and the unsqueezing will be done automatically.
To use it enable 'framing' to get a preview in liveview. It is working in regular video mode(NOT x3 zoom mode). Set bitrate to 10bit and ratio to 2.35:1 for continuous recording.
Please report back how it works and feel free to share content recorded.
By the way. Do not miss out on this progress!
https://www.magiclantern.fm/forum/index.php?topic=17695.msg209985#msg209985
Build for download in following post:
https://www.magiclantern.fm/forum/index.php?topic=9741.msg208959#msg208959
MLV test file:
https://bitbucket.org/Dannephoto/magic-lantern/downloads/short_M01-0722.MLV

TODO:
Focus pixel map(dfort is looking into it).
This mode actually also works in mv720p which yield 5x1 squeeze. Will check if it´s actually useful in real life or not.
One minor nitpick: if you zoom in at 1:1 and look at diagonal edges, you will see some jagged lines (as if the columns were somehow reordered).
These artifacts are exactly what I'm talking about in these comments:
/* 3x1 binning (bin every 3 lines, read every column) */
/* doesn't work well, figure out why */
case CROP_PRESET_3x1:
/* ADTG2/4[0x800C] = 2: vertical binning factor = 3 */
/* ADTG2[0x8806] = 0x6088 on 5D3 (artifacts worse without it) */
adtg_new[2] = (struct adtg_new) {6, 0x800C, 2};
if (is_5D3) {
/* this register is model-specific */
adtg_new[3] = (struct adtg_new) {2, 0x8806, 0x6088};
}
break;
I don't remember whether the artifact is present in the 3x1 mode from the
bleeding edge 700D crop_rec; maybe Bilal can double-check, if he's still around. The issue is present on 5D3 too, but I wasn't able to fix it there.
Anyway, my suggestion for a 4K stretched mode would be to rotate it at 90 degrees (i.e. take Bilal's
1736x2214 in 1x3 and attempt to squeeze it even more, to get larger heights, up to 3478).
Why? That way, you will not skip any physical pixels on the sensor; all Canon cameras I've looked at appear to be able bin pixels horizontally, every 3 columns. Since this camera is not able to bin in the vertical direction, you'll just read out every line.
With the bleeding edge 700D crop_rec, you should get 1440x3478 at 19.2 FPS:
[crop_rec] source: 1736x1160 @ 23.976 fps
[crop_rec] target: 1440x3478 @ 19.194 fps
[crop_rec] ylimit: 1398,3478
[crop_rec] timerA: 528 -> 446
[crop_rec] timerB: 2527 -> 3738
[crop_rec] delta : X-296 Y+2318 [z=1]
From there, you might be able to reduce the horizontal resolution even further; I just don't remember where the hardcoded 1440 limit comes from, and have no way to re-test it atm. For 24 FPS at full height, you need to go down to about 1080 pixels horizontally, but I'm not sure whether the hardware will accept it:
[crop_rec] source: 1736x1160 @ 23.976 fps
[crop_rec] target: 1080x3478 @ 23.976 fps
[crop_rec] ylimit: 1864,3488
[crop_rec] timerA: 528 -> 356
[crop_rec] timerB: 2527 -> 3749
[crop_rec] delta : X-656 Y+2318 [z=1]

Expected result: much lower aliasing.
If that works, you can try to go even further; for squeezed 16:9 you would require only 3478 * 9 / 16 / 3 = 652 pixels horizontally. If the hardware accepts it, that should give some half-decent rolling shutter, too.
Happy New Year!