I had a wee look at the code (though i've got to be honest and say 90% of it is well over my head), and it seems that all the hud gauges are specified simply by their co-ordinates - i.e. just numbers. So i'd guess you could just modify those by using a variable instead, and defining it somehow (maybe in a table).
I was thinking a model might be a lot more visually impressive, and could maybe (?) have some basic form of lighting - basically there's already some pof viewing support for the target box, so how much of a hit would it be to add an extra 800-ish pof-based cockpit model? Personally, I'd think a bitmap would be easier, but it is a bit more crude IMO.