I was reading one of the many pages online about adenium, and found this;

Grafting
Grafts of Adenium multiflorum on Nerium oleander roots are well documented and are widely marketed as container plants in Europe. Such grafted plants can be watered more frequently without fear of rotting of the roots.

http://www.plantzafrica.com/plantab/adenium.htm

So, I am thinking about buying a young oleander, and giving this a try with an obesum.