The open-source IllumOS code in FreeBSD had fallen behind Linux ZoL (ZFS-On-Linux). FreeBSD adopted ZoL to keep up with new bugfixes, enhancements, etc.
As suggested, one path is to port Rockbox firmware for your MP3 player. However, since you do not have any prior experience in embedded domain, it will not be a easy path.
I would suggest that you try using the Arduino or RaspberryPI hardware. Both of these platforms are widely used, so you would be able to easily get help if you get stuck. Also, these are 'simple' platforms that are widely available, so the learning materials available is unmatched.
Building a simple project like a weather station or clock will keep you focused on a "actual" target/goal.
https://www.freebsdnews.com/2019/01/10/zfs-on-freebsd-zof-is...
So, for open-source, ZoL is the 'mainstream' ZFS.