No one knows? :(

Not specifically for cup and saucer seeds, but I have been on a rampage collecting seeds from purslane and Madagascar periwinkle, almost of them were green and terribly unripe. The best way to do it is just remove the entire pod from the plant and put it in a paper bag, then store in a dry place. the pods will ripen on their own and burst open in the bag. When they are done, just separate the chaff from the seeds, and save them for next season. This has worked for all types of seeds for me, as long as they are given a chance to dry and ripen.

Thanks stephanotis! :)