Indeed, when I comment out my overlay:
$ grep -ac ZSTD /tmp/inspect1/u-boot-rpi3.bin
0
$ grep -ac BTRFS /tmp/inspect1/u-boot-rpi3.bin
0
So it seems like the overlay is working, at least for the initial SD image. I don’t think anything gets overwritten there by default after a nixos-install, but I’ll check that out next.