Thank you for the encouragement and pushing me in the right direction!
It might be as easy as putting "System.exec" in there. For now i am not getting any further as:
/usr/share/kodi/addons/webinterface.default/js/kodi-webinterface.js
is on a read-only filesystem.
Currently i am trying to find out how LibreELEC is mounting filesystems whatsoever, as fstab is empty.
EDIT: OK, LibreELEC uses udevil, and i have seen that there are conf files and that i could mount manually, but for now i am not able to find out for this file a) on which filesystem it is and b) where that filesystem is mounted from.
EDIT2: Ok, i have found this. Device seems to be read-write mounted, maybe the filesystem is not.
LibreELEC:/ # udevil info /dev/disk/by-label/STORAGE
Showing information for /org/freedesktop/UDisks/devices/mmcblk0p2
...
has media: 1
is read only: 0
is mounted: 1
mount paths: /storage
...
label: STORAGE
Maybe i there is a way to just mount this a bit differently?