Bear with a relative noob here please.
I've done a fair bit of research behind why MP3Tag has implemented player-specific Rating tags for ID3 POPM fields, and it does seem an ugly but necessary state of affairs, as much as we'd like to have a Rating tag that worked across the main players that just doesn't seem possible without a dedicated coding effort that probably isn't merited by the apparent lack of general interest.
However in my little bit of noodling around I seem to have come across something that works for at least a couple of decent players, at least for FLAC files, and I'm posting what I've done so far not only so that it might help others interested in the topic, but perhaps MP3Tag could continue to lead the way in pushing this tech subculture forward towards greater interoperability.
In checking out various players, I've come across two Rating-capable free apps that are "portable" in the windoze sense of not requiring installation to the registry, just copy the folder to a removable HDD or flash drive whatever and all your settings come with you. The first is Foobar2000, and the other is MusicBee; these are on opposite sides of the spectrum for philosophy, intended audience, customizability vs bells-and-whistle features, but both seem pretty darn good.
In checking out how Rating is implemented, MusicBee is fairly straightforward at least with FLAC - it creates a Rating field and uses a 0/20/40/60/80/100 scale for 1-to-5 stars. I seem to have come across this scheme before, if anyone knows where please inform, it seems cleaner to me than the ones based on 255 - does it correspond to any of MP3Tag's options?
(as a side note MB replaced my ALBUMARTIST tags with ALBUM ARTIST ones and inserted an ENCODING field without asking, but that was easily worked around with MP3Tag's great mapping and scripting features.)
In looking at Foobar2000, that's got quite a few very flexible and therefore confusing options for implementing Ratings, and the trend there seems to favor library/database storage rather than in-file tagging, but I came across a "quicktags" plugin that uses tags and is simple and flexible. The standard howto recommended a simple 1-5 scheme, but I adapted it for MusicBee's fixed 20/40 etc one. In case anyone's interested here's the column display formula (those are unicode stars in there in case they don't display right on your screen):
I bound updating the field to keystrokes Ctrl-0 through Ctrl-5 for FB2K; unfortunately MusicBee seems to require using the mouse.
Now comes something at least tangentially relevant to MP3Tag - a display formula to do the same thing as the above:
I could of course just display the numeric value of the tag, but that would be too easy and therefore boring
If anyone feels even a tiny fraction of my excitement about this - a Rating tag that works in multiple mainstream quality player apps as well as being simple and straightforward enough to lend itself to easy conversion to the more screwy ones - then do please feel free to extend on this with suggestions.
I will at some point need to convert my FLAC files to MP3 for portable-hardware use, and of course it would be way cool if I could continue to apply and make use of ratings from there, so if anyone has any ideas for that, especially in the Android environment, that would be very very cool.
Much as I hate the Big Brother Apple has become in the music scene, an iTunes/iOS conversion routine would probably have the greatest appeal to the community in general, but it seems that'll have to be done via a one-time import from the WMP/MM formats (they are compatible right?) and generic Windows Explorer compatibility would be most useful (even if read-only).
I doubt if I'll ever go back to Winamp.
Finally, thanks to the dev(s) for such a wonderful project as this, MP3Tag's a tremendously useful and even elegant tool, refreshingly respectful of the end user in a pretty wild-wild-west area of software.