This here is supposed to make it fullscreen without any complications (which it does for me, but I'll have to check in an Android 4.1 VM):
https://github.com/koreader/android-...nifest.xml#L20
(no status bar, no system icons, no nothing)
There's also a manual toggle you could give a try in Settings → Screen → Fullscreen or something like that, though it might be slightly broken.
https://github.com/koreader/android-...y.java#L60-L75