Which filesystems support embedding?

[Dark Penguin]

Greetings!

I want to have several small partitions exclusively for hosting grub2
bootloaders for massive multibooting (through chainloading). I install
many distributions on a single btrfs partition (multiple subvolumes),
and create many small partitions for easily managing their bootloaders.
(Also, EFI is unusable on my hardware, so this is the easiest way.)

However, I can not find out which filesystem they must be formatted in.
I've tried all common filesystems, and none of them "support embedding".
I'd be fine without formatting them at all, just installing grub2 on raw
partitions, but that doesn't work either, even with --force parameter.
(Also, I don't think os-prober in my main installation will find them
this way). The only applicable filesystem seems to be btrfs, but it's
hard (if even possible) to make btrfs partitions only few megabytes in
size, as btrfs allocates space by big chunks.

Which filesystems can I use for this case? I don't care if the
filesystem itself is unusable after this - I only need it as a
workaround to have grub2 installed and detected by os-prober.


-- 
darkpenguin