Oki as a starting point you can use this texture for the ceiling's emission:
*edit: awoops bitdepth is wrong, needs conversion

ah no it has an alpha layer
I'm editing mats by hand, I didn't do such emiters since I stopped using mental ray and final gather.
** no way I've got a
luminance() unhandled error trying with textures, even jpg
I'll try another approach, a box with openings and sun or uniform background. Sounds bad but could do.