I have an Arch installation with /boot/efi as the Fat32 partition, with /boot part of btrfs /
I would like to install NixOS likewise, but the installer balks that /boot is not mounted.
How can I bypass that issue? As I am quite happy with with my root drive partitioned the way it is.
I don’t know if it’s possible with systemd-boot, but with grub, you can set the boot.loader.efi.efiSysMountPoint option to whatever mount location you want for the ESP.
That’s not a grub-specific option since it doesn’t have grub in the name 
I know that, but I don’t use systemd-boot myself, and I’ve heard conflicting things about whether the menu generator for it actually respects that option. Don’t want to over-claim.