My S. sessilifolia [Madagascar] has flowered, I'm collecting a few seeds, and I've managed to root it twice by layering! Cuttings didn't work, but I must admit I didn't give that much chance due to lack of plant material.
The first attempt at layering took a few months, presumably due to colder weather. The most recent attempt was much faster, and one can see the mass of roots and new shoots in the photo. The stems are vertical, so I hold them down in soil with rocks.