If this is a standard Blinn-Phong scheme, then the specular reflection strength is computed by the fixed pipeline / DirectX equivalent of
specular.rgb = (gl_FrontMaterial.specular.rgb * gl_LightSource[0].specular.rgb * pow(NdotHV, gl_FrontMaterial.shininess));
(this is the OpenGL specular equivalent of the fixed pipeline function, but you get the picture).
You can't influence the light source without influencing anything else, but you should be able to set the material constants (material.specular and material.shininess) in the 3d model you're rendering - they're usually not hard-coded but a property of the model, which means you don't need to adjust the binary to change them.
Also, a model may have a specular map where the reflective areas are marked, that's just a texture you can edit to increase reflectivity.
So I would guess that you can in fact modify it, but I'm not really familiar with the Orbiter particulars, I just know how 3d rendering works in general and in OpenGL in particular.