one other idea would be to use bitmaps on the boxes. like, if you have a box with "B" on it, make a bitmap that has the "B" all black and the surrounding area white. use this in the self-illumination slot. if the BG color you've used behind your B on the actual diffuse map is a REALLY light color, like white or a slightly yellowish white, it will light up nicely.
in fact, using this technique with Richard's specualr bloom tutorial in the Tutorials thread might look pretty cool!
if you've already finished this image, post it so we can check it out!