Seems like I got it to work, for anybody wondering the same thing, here is an example.
// Where GeomInstanceArray[] is of type domInstance_geometry_Array
// Fetch the Geometry instance from an array.
domInstance_geometry* geomInst = GeomInstanceArray[m];
// Get the geometry
daeElement* Elem = geomInst->getUrl( ).getElement();
domGeometry* GeometryElem = (domGeometry*)Elem;