Actually this comment is from August 2012..
I haven't looked into the minirt for a while - maybe btrfs is there already? Otherwise, it's "just" to get the right kernel module, add it in and modify init.
1 and 2 should be quite straightforward - 3 I have no knowledge about, but if it amounts to substitution of compression methods, it might be OK. A bit unsure about unionfs problems though.

For my own use, I think I will stick to squashfs.