After a week of trying around, I suddenly found a very simple way to achieve, what I was looking for.
Using a QPainter and it's methods instead of the QGLWidget's renderText()
simply makes text clipping work:
QPainter *painter = new QPainter();
painter->begin();
painter->setClipping(true);
painter->setClipPath(...); // or
painter->setClipRect(...); // or
painter->setClipRegion(...);
painter->drawText(...);
painter->end();