Wait, completely no model changes? That could come out clunky and hard to use. I thought you'll use glowpoint types.
As for the format, I'd like something like:
$Type:
$Index: (which glowpoint type number we're talking about)
$Thruster Bitmap: (used instead or in addition to index, name of the thruster bitmap that, when used either in the "main" or tabled "maneuvering" thruster will emit this kind of light)
$Color: (self explanatory, 3 values RGB. 3 zeros disable the light. If absent, the color will be based on bitmap's histogram)
$Alpha: (bitmap transparency percentage, setting it to 100 disables the bitmap completely)
$Intensity: (light intensity, gives finer control than color)
$Light Cone: (number of degrees from the normal the light will be radiated)
$Bitmap Cone: (number of degrees from the normal the bitmap is fully visible)
$Flags: ( "No Self Lighting" "No Obscuring" "No Flicker" "Start Off" ) (the first one disables the glowpoint casting light on it's parent model, the second one causes the glowpoint to be rendered on top of the model, the third one disables flickering when the ship is damaged and the last one makes the glowpoint off by default, to be enabled in FRED)
That should be it I think.