Hi Maren,
the problem with Cephalanthera rubra is not how it should be sown, but how it should be established in vivo.
It seems to rely heavily on it's endophyte-partner in nature. Although there are 1 or 2 people who had success with it, I guess that this is an orchid you will never find in commerce because of it's special needs.
It is more difficult to establish even than the most difficult Cypripediums.
BUT after it has been established, it is quite easy....