I know at least two (fairly widespread) ones that don't: WMP and iTunes.
WMP also does not show the BPM field whereas iTunes refuses to tell us the LANGUAGE...
So even if you have filled MIXARTIST it would be information that is lost as it is fairly likely that the player won't show it.
If, in the meantime, you devise a unique naming pattern for the data you want to look for then MP3tag will help you to transfer it to the correct fields, once this has become a widely implemented feature.