My experience with LH (actually AC flight number, but LH metal) is that you can't book a specific seat. Other Pax, who booked direct with LH could. AC staff at YVR says they get lots of complaints because of this.
Having said that we got put into two seats together near to where I would have chosen them myself so it wasn't a big deal. Seats reasonably comfortable and service OK.
This was 18 months ago, things may have changed.