Nananthus pole-evansii is another small south african succulent. The seed for this plant was from Nels river dam. These have been found to be cold hardy for us here in Pueblo. These plants make dense mounds similar to Nananthus transvaalensis. 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.