Hi, and welcome.
I've seen variagted plants of M. marksiana on sale on eBay from time to time, but I can't ever remember seeing seed. Is there such a thing, I wonder.
Variegated plants often occur quite naturally as a result of some defect (?) in their make up. Often they are found in small numbers in large batches of seedlings.
I wonder what would happen if you propagated a variegated plant and obtained seed, would this be likely to produce only variegated plants, or would they be just as likely to be "normal" as ordinary seed?
Sorry I can't help more.