Ollee does that by replacing the original PCB. In this project the movement is kept original.
I recall the blog n-o-d-e did successfully put an NFC chip inside while also retaining the default PCB, so there is at least some free space in the case...
It is relatively easy (currently) to take a playlist and back it up as a plain text file.
CTRL+A, CTRL+C from web or desktop puts a newline separated plain text list of songs on your clipboard.
Main risk is that the list is in the format of "https[:]//open[.]spotify[.]com/track/[song-id]", so Spotify may break the lookup of these IDs in the future.
I recall the blog n-o-d-e did successfully put an NFC chip inside while also retaining the default PCB, so there is at least some free space in the case...
https://n-o-d-e.net/casio.html (0:56)