That seems ill advised, one of the benefits of btrfs is it obviates the need for lvm and mdadm.
I guess in your case you have a more stable raid5/6 opportunity, but you're losing many of the raid benefits present in btrfs natively. I'd also imaging it could be slower or introduce IO issues others haven't tested. Though I really have no idea, never seen anyone do that before.
It's been great for our use case - we've been using it for about 5 years like this. We use it for rsyncing data onto as a backup (not the only one!), making daily snapshots. md is more flexible than zfs's raid, but less flexible as native btrfs raid.
