Tabs seem to propagate better via root cuttings from what I've been able to learn. I have a friend who says iboga will propagate from leaf cuts, but the ones he sent me all failed, and when pressed he admitted he'd never done it himself. Real ibo growers will tell you that they propagate them either by seed or root cuttings, though I have no details at this point.
My Tab. divaricata (admittedly not a difficult species) will easily propagate from roots. The way I discovered it was I had a potted plant that sent roots through the bottom of the pot into the soil underneath. As winter approached I brought it into the greenhouse for protection, breaking off all those roots. The next spring new plants emerged right where the pot was sitting the previous year. Maybe you could try something similar.
I've been hesitant to buy T. sananho, because the vendor I've been looking at only offers unrooted stem cuttings, and to my knowledge stem cuts from the Apocynaceae are notoriously difficult. Will yours be rooted when you get it?
I already asked Alice.