That is correct, you will need a second mesh, just a few centimeters above the first with the illumination map applied to the second mesh. This mesh could be low poly too FYI. Or you could just make a mesh and cut out all the illuminated areas so the rest of the mesh isn't included, that might work best. If you choose to just duplicate the first mesh, here is what I suggest. Copy the original mesh, select all faces and extrude group a define distance. You'll have to experiment, but I've found this distance can be quite small, a cm or even less.
Use opacity map channel as illumination, (white=illum, black=non-illum), and also "alpha_masked" to true so only the illuminated areas are visible. In the day time the illumination mesh will be completely transparent, so you won't notice it is there. Angrypig said the real illumination map is nearly done, so hopefully this won't be necessary in the near future. Send me a PM if you need help, and if you send me your OTX I can edit the material to work correctly if you require.
Best regards,
Uriah