I haven't worked with the FFP for a while, so I did a quick test now and it seems that the FFP lighting will never (*) produce enough light for the surface to get brighter than its normal texture color at 100 % intensity. It's quite weird, but even if you combine full while (0xFFFFFFFF) ambient color with as many directional lights shining at the surface from the same direction as you want, you will never get anything brighter than the original texture color. Also emissive color of the material doesn't help.
Maybe there are some combinations in SetTextureStageState that would change it, but I honestly don't know.
If you don't manage to get the desired look by playing with ambient and diffuse colors, you will probably have to use shaders. I don't think that would be such a problem with low-end computers. Well, yea, you would increase your HW requirements to GCs supporting shaders.
(*) The only exception seems to be specular colors, those (of course) get added "on top" of the diffuse and ambient lighting. But I don't think specular would work in your case (ortho view, directional lights, flat terrain).