The ITUNESOWNER field actually refers to the ownr MP4 atom (search for this in your HEX editor). I've just ran some tests with v2.80 and it gets updated correctly.
What doesn't get updated is the DRM data that also references the name of the files' owner. I have no plans to mess with this data, since Mp3tag would then be very quickly on the grey-to-dark corner of the software spectrum 
Kind regards
– Florian