It's been a fun journey, but I've finished the first version of my C music library for the Playdate! It can play back most S3M files (minus a few rarely used / poorly supported effects).
I'm not sure how performant it is compared to other Playdate music libraries and such. It's been able to play all of the 16+ channel complex demo scene music I've thrown at it, and the heaviest of those use around 50% CPU. However if you keep the number of channels relatively low, it should leave plenty of CPU cycles for a game. I do intend to use this in the game project I'm currently working on.
I made a repository for it here: GitHub - briankendall/playdate-tracker: A music tracker for Playdate that can play S3M files
The repo includes a little app that shows off how to use the library, including some of my favorite scene music I used for testing, and allows messing with the music using the crank. I was surprised by how much fun that little feature is! Here's a video if it in action: https://www.youtube.com/watch?v=PXwX89gtiDY
Anyone is free to use this in their game. If you find it useful, I'd love to hear about it. 