Answering my own question here. Tested by tagging my songs with different specifications (ID3, APE and Vorbis comments) for ratings and Neutron player seems to be supporting only the Vorbis RATING.
But boy, if this is confusing: While it seems to be that RATING is not in the specification of Ogg Vorbis, according to Wikipedia article:
Most applications also support common de facto standards, such as DISCNUMBER, RATING, and tags for ReplayGain information. Ratings are usually mapped as 1-5 stars with 20,40,60,80,100 as the actual string values.
So, if that is now standard enough, and @adrian is willing to implement that to mediafile.git, I’d be grateful.
Maybe do some research what other software uses for a RATING tag?
If this will make a difference in building a business case, I can start doing that and list ios & android apps in here that utilize this de facto standard.