To have that mount propagate to the container, you’d need the path on the host to be marked as MS_SHARED.
Something like this should work, this assumes that /mnt in this case is shared with your container:
mount -o bind /mnt /mnt
mount --make-rshared /mnt
lxc-start -n your-container
mount /dev/sdX2 /mnt/blah
In this case, that blah directory and mount should propagate to the container thanks to the bind-mount and rshared flag.