Booting from btrfs subvolume

As this is my first post here: hello everyone! :)

I'm slowly migrating to AL and have some struggle with booting from btrfs subvolume. My goal is to have one big partition, totally owned by btrfs and rest of the dividing should be handled by subvolumes.

What's working:

1. One big partition on msdos/mbr with no subvolumes at all.
2. One big partition on gpt with no subvolumes at all.
3. With separate /boot partition on msdos/mbr with no subvolumes at all.
4. With separate /boot partition on gpt with no subvolumes at all.
5. With separate /boot partition on msdos/mbr with btrfs subvolumes and with set-default subvolumes set to @.
6. With separate /boot partition on gpt with btrfs subvolumes and with set-default subvolumes set to @.

What's not working:

I was going to create bug report/feature request, but thought that I might be missing some obvious step that's escaping my attention -- hence the topic. Here are my steps when I try to boot from btrfs subvolume:

1. Booting AL & running quick setup via setup-alpine -q.
2. Install parted and btrfs-progs with all dependencies necessary.
3. Using parted to end up with following partitioning:

Partition Table: gpt
Disk Flags:
 
Number  Start   End     Size    File system  Name  Flags
 1      1049kB  3146kB  2097kB  btrfs              bios_grub
 2      3146kB  21.5GB  21.5GB  btrfs              legacy_boot

This is an example for GPT partition table, but it doesn't work on MBR/msdos either.

4. mkfs.btrfs /dev/sda2 and mount -t btrfs -o rw,noatime /dev/sda2 /mnt.
5. cd /mnt and btrfs subvolume create @ (yes, naming convention taken from Ubuntu, get over it).
6. cd and umount /mnt.
7. mount -t btrfs -o rw,noatime,subvol=@ /dev/sda2 /mnt
8. Once that's done, one may confirm via grep btrfs /proc/self/mountinfo that indeed subvolume is being used.
9. mkdir /mnt/boot -- even though I'm not using separete /boot partition, this step is still necessary (bug? feature?).
10. Installation ongoing: setup-disk -v /mnt.
11. Once installation is done, my understanding is, that there are three steps necessary (in case of GPT at least):
a) edit /etc/fstab and add subvol=@ to the mount options for the root partition.
b) edit /boot/extlinux.conf and add rootflags=subvol=@ in the APPEND section.
c) dd gptmbr.bin to /dev/sda.

The problem is, that it won't boot. I tested this stuff out with using separate /boot partition and figured, that the only way to boot it is to use btrfs subvolume set-default #ID (taken from btrfs subvolume list command) /path -- but this way the whole point of using subvolumes is lost, one can just not make the whole effort in the first place (also this way /etc/fstab for the root partition is omitted and it doesn't matter whether there's rootflags specified in the extlinux.conf).

For me it looks a bit like rootflags is not being respected by the bootloader -- which is weird, cause this is the way it should work and is working in other distributions (RHEL: https://bugzilla.redhat.com/show_bug.cgi?id=1033764, Slackware: http://dvaerghoffmann.tumblr.com/post/33722281147/install-slackware-14-btrfs-extlinux-ext2).

Is rootflags available in extlinux provided by AL? Am I missing any step after editing /boot/extlinux.conf file? Should I somehow update the config differently? Any ideas anybody? Maybe someone already have it working?

All the best
-Hadret