cat /dev/zero works for btrfs if you turn off compression first:

btrfs property set PATH compression none

For example:

mkdir -p btrfs-zero
btrfs property set btrfs-zero compression none
cat /dev/zero > btrfs-zero/zero ; rm btrfs-zero/zero

Credit: https://unix.stackexchange.com/questions/44725/clear-unused-space-with-zeros-btrfs