CONTENTGROUP is already there. See the help section Keywords for SetAttr META. If you need to write tags not supported by Opus you can use ExifTool, which gets along nicely with Opus.
i found this in post with no answer .....
how do I add tags containing multiple word using SetAttr META tags? I tried #1 SetAttr META tags:word1 word2 #2 SetAttr META tags:word1 word2; #3 SetAttr META tags:'word1 word2' #4 SetAttr META tags:'word1 word2'; #5 SetAttr META tags:"word1 word2" #6 SetAttr META tags:"word1 word2";
#1 and #2 have lost word2 somewhere in the process. #3 and #4 did the same with setting the tag to 'word1 at the same time. #5 set the tag to "word1 word2 (including one of the unwanted double-quotes).
and #6 set the tag to βword1 word2β (including both of the unwanted double-quotes).
why I am at it i will ask how to add more then one grouping lets say you have Studio Album and you would also like to add Christmas Album to the same files using CONTENTGROUP
thank you
the music program i use uses a ; as a separator so the same album shows as Studio Album and Christmas
not quite there yet for two types yet SetAttr META "contentgroup:Studio Album";Christmas Album with this i get the two groups but Studio Album shows as Studio Album" with a " on the end