Not really a solution:
I found that the image to flash to the system partition (i.e. "ubuntu.img") is a usual ext4-partition as opposed to the former "system.img" which is an Android sparse image.
So I mounted it, made it writeable, changed "GridUnit" in /etc/deviceinfo/devices/halium.yaml, saved it and flashed it via fastboot. (I don't know how to do it if you're unlucky to run a windows machine).
After rebooting*) the scaling was altered as intended.
So this is a way to change settings until the Tweak Tool for Lomiri arrives.
*)Fortunately the Lenovo tablet took only a few efforts to make it