Music players reading of tags & comments

Players playing mp3s with ID3v2.3 tags, I am assuming that they read the
ID3v2.3 internal naming and transpose it to the label, such as

Players playing flac files would read the label definition such as
Artist = Rolling Stones with out any reference any internal naming like TPE1 = ARTIST?
But will they be unable to read any non standard labels like
Label = Mudpie, value and field = %contentgroup%.
Would they be able to read it as CONTENTGROUP and its value?
or it not at all and if so would the song fail to play?

depends on player. Generally the answer to above is yes for me (using foobar2000 and LMS for Squeezebox players)