GRUB2: /boot as BTRFS subvolume

Yes. Myself successfully moved /boot into the BTRFS system as a subvolume. No special consideration necessary - just don't forget to reinstall grub and rebuild the initramfs.

edit:

Will require running grub2-mkconfig -o /boot/grub2/grub.cfg after every upgrade involving a kernel (linux, memtest, etc) or during/after a fedora upgrade (fedup) as grubby does not support updating grub.cfg on btrfs subvolumes. There is a patch available [#864198 - see attachments] which hasn't been integrated because it's a hack. With kernel-install and BootLoaderSpec, grubby seems to be nearing end-of-life, so I don't find it objectionable.

TLDR:

Requires updating grub.cfg manually every post-kernel upgrade, which is painless unless you've directly edited it.