I have heard sung here in the US Northwest a splendid chord song called, I think, The key of R Flat. It purports to be a key only known to the singer, and it uses all the chords in the harmony book very cleverly- R Flay Major, R Flat Minor, RbDim7, etc, using them as they are called out in the lyric. Someone must be able to cite the writer and perhaps a performance.
Carter B