Can you account for radiation pressure by modifying the orbit? It's a constant radial force. Searching for Lagrange points is searching for points where (in the rotating reference frame fixing the Earth) the combined gravitational forces of Earth and Sun are zero. Just add in the radiation pressure and you should get a Lagrange point slightly closer to the sun.
Buut I haven't done the math so this is speculation.
Buut I haven't done the math so this is speculation.