Hmm...the Stemme may not be a true motorglider by that definition since it has a 115hp engine. Of course, different aviation authorities have different definitions of a motorglider. In the US, the language is very vague. If the aircraft's free flight does not principally depend on the engine and the pilot's intention is to soar, then the aircraft is considered a "self-launched glider". Personally, I think that is a really lame definition, since I've seen video of a Pawnee tow-plane jump into a wave after glider release and shut the engine down! Any airplane will soar with enough lift.
I think in the UK a glider is defined by the wing loading. If the aircraft has an engine but is still below the wing loading limit then it is a motorglider. Is that right?
But as far as flying a motorglider in mountainous regions, well, I would use the mountains to my advantage. I mean this is still a sailplane and ridge lift or waves could likely be found to get the alt needed to pass.
Also, I hear you as far as density altitude, but I soar on very hot and humid days here in Florida in my Pilatus and I easily find thermals that can give me 300ft/min climb rates to cloud base. I've never soared in Africa, but I'd imagine the conditions must be pretty similar to the southwestern US, which has some of the best soaring in the country. The temp gradients from night to day produce some great thermals once the sun starts heating the ground. The climb rate may suck until the motorglider can find a decent thermal, but after that who cares about the density altitude so long as mother nature is providing the climb rate.