It is a rare and (often heard, seldom heard) song

Either word might fit. To say there will be a playlist of her rare and seldom heard songs is grammatically correct; it will include those of her songs that are rare and seldom heard. On the other hand to say there will be a playlist of her rare and often heard songs would usually be understood as saying the playlist would include rare songs together with often heard songs.

Arguably though, to speak of "her rare and often heard songs" is contradictory, and the sentence should speak of "her rare songs and her often heard songs". For this reason, if you must insert a single word I would go with "seldom".