After more research, it looks like you can just define a combine callback (GLU_TESS_COMBINE) and calculate a proper Z coordinate (radius away from center of sphere). I've never used GLU, so no promises. If that doesn't work, you could always just manually tessellate. Have you tried that and run into problems? I suspect there might be more people familiar with tessellating than using GLU's tessellation process. If you haven't tried, it's probably a lot easier than you might fear.
After tessellating, you might need to make sure the sphere doesn't bleed through. You can do that by making the continents higher than the sphere (radius + fudge factor away from center of sphere). I'm not sure just prioritizing continent data will get rid of all the bleed for this specific situation, but I guess you could try. You can either turn off depth buffer or do it in the shader or render the sphere to texture and use it as a skybox or any other way. But I'm betting for this situation you'll wind up just protruding the continents from the sphere.
