I am new to this and wanted to know why you don't see more planes with backlit gauges? It seems to me that alot of planes could benefit from them.
Are you refering to backlight gauges on the 2d panel or in the virtual cockpit?
For gauges on the 2d panel, the individual elements within the gauge may be coded to use either LUMINOUS or BRIGHT tags.
LUMINOUS attribute is additive, and derives it's specifications from the panel.cfg entry cleverly named Luminous=R,G,B (where R,G,B are units of color).
BRIGHT attribute effectively doubles the 'brightness' of the bitmap uniformly, although one can also specify a replacement bitmap to which they've applied a "Lighting Filter" in Photoshop or PSP.
If you are speaking about virtual cockpit backlighting, that is a 'feature' of the model itself, and the designer must specify that a lightmap be applied to the Material properties used for the gauge polys. If the modeler fails to take the necessary steps to allow backlighting, there's nothing that anyone can do to "force" the issue.
Now, FSX appears to offer a new method of virtual cockpit backlighting that I haven't figured out yet... but, the 'old method' I developed three years ago will still work.
For details on VC Backlighting using Emissive Lightmapping, see my ancient tutorial at
http://www.freeflightdesign.com/tutorials.html