Rabiea albipunctata is another small south african succulent but with a large flower. The seed for this plant came from Bill Adams. These plants make dense mounds similar to Nananthus. Plants have taken -8F with light snow cover and -2F with no snow cover. Plants also flowered on and off all winter long and have shown to be very hardy in my opinion.
I do not guarantee these in a zone 5/6/7 because everyones growing conditions are different but this would be a good trial plant to grow in your cold outdoor garden. Try to plant outdoor 90 days before 1st hard freeze.