Yes, I know about MuseScore and the "plugin" you mention - it contains a call to a web service that runs abc2xml.py, and then uses the built-in MusicXML convertor. I am not a fan of web services for free software (- for commercial software we will probably have to accept it as a way of payment), all the more so as abc2xml.py is freely available, and so is the required Python environment for all platforms. Other people may prefer web services to installing Python or Java etc., either for simplicity or for security concerns.

Anyway, abc2xml.py (not to be confused with earlier attempts of similar names) is quite good for my demands, as I mentioned on the other thread. So is its converse xml2abc.py. I am not so sure about the MusicXML support of MuseScore; I remember that it had severe bugs a while ago. I do not use MuseScore regularly.

As for the standard compliance of the two tools, I don't know of any problem, neither with ABC nor with MusicXML. Still, it cannot possibly be any less "volatile" than the standards themselves. (ArtfulCodger tells us that MusicXML is not very well standardized either, though I have never heard of genuine problems that were not clearly seen as bugs. I guess any doubt can be resolved by experimenting in Finale - not a good way of defining a standard, to be sure, but effective.)

On some more reading in "abcusers", I am not convinced that we will soon see a good result. I cannot judge about the quality of the discussions and proposaly, but unless my sample was extremely unrepresentative, JFM and the other major programmers are scarcely involved. Chris Walshaw may have given up the hope even to repeat the relative/partial success of ABC 2.1. If neither he nor others offer a good standard to us, Mudcat must become active in the way I described above.