live-boot doesn't currently have a way to pass mount options for the
live system's backing store. I use plainroot off a btrfs subvol, which
requires the subvol=${subvolname} mount option to be passed.

I'm currently using the attached patch to make this work, but it's not a
general solution to passing mount options for live-boot backing stores,
it explicitly only supports my plainroot use case (but I'm unsure of how
much utility there would be on supporting mount options for other use
cases).