Note: This is an old method with very low bitrate. Suggested approach is via Vasile's or NX-KS modpack.
First see procedure of running shell scripts from SD card as explained here: Running arbitrary shell scripts from SD card
In nx_cs.adj file put:
sys_param movie size 2560_1440_30p
shell script /mnt/mmc/killdfmsd.sh
In killdfmsd.sh file put:
sleep 1
killall dfmsd
Killing dfmsd daemon enables the use of touchscreen.
Now, when you press the DEL (Trashcan / Video preview) button and enter the video mode you can see that the resolution icon id showing MJPEG (but it's not MJPEG - it's just the last icon it found when searching for the right one that does not exist).
Here is what ffmpeg says about the video stream
Stream #0:1(eng): Video: none (hvc1 / 0x31637668), 2560x1440, 11280 kb/s, 29.97 fps, 29.97 tbr, 120k tbn, 120k tbc
So, very low bitrate - barely usable (it was a static scene, but still).