So, having a composer search hasn't worked in the past. What about having composer verification and letting them add the songs they've worked on to their profiles? If the composers are verified beforehand by submitting proof of composition and identity then they can add their songs (perhaps a further proof step here?) to a verified playlist of what they've done?
The onus of proof is put back on the composer then and creates less work for Spotify and the label but still let's composers showcase their work.