This is exactly the way I tried it first.
But sshd relies on many different files and folders which I thought are easier to provide in a chroot'd environment and do not conflict with the existing empeg things.
Maybe it would somehow work the way you proposed but I had no success.

Is there really now way to mount, link (e.g mount_union, mount_null) or whatever a directory from outsite the chroot'd environment? (I know the purpose of a chroot'd environment is to jail the user in there)