Ok, another 4 hours wasted on the most crappy SBC platform OMV is capable to run on
These fixes should've done the job. At least motd (login greeting) is correct now and also generic installed packages are only pulled in from Debian but not raspberrypi.org any more.
@ryecoaaron or anyone else: please test again: OMV_4_Raspberry_Pi_2_3_3Plus.img.xz