Yes, some decades ago (i.e. when Apel was writing), it was common to transcribe mensural music at a value reduction of 4:1 (i.e. 3/4 for Circle); now, 3/2 is a more common transcription level, and most specialist performers prefer to read either from original note values (if transcribed), or from original notation, if the manuscript isn't filled with errors like the examples I'm currently dealing with. I'm working on a bigger project that may involve generating multiple versions in different types of notation from the same source file; from that point of view, it would be more convenient if the mensural sign was more closely attached to the meaning. In the meantime, at least I can get the symbol, so thanks for that!
Thanks for the point about the clefs! Now I know why it works.