This is the current state, in terms of code it gets away with being under 200 LOC (Nasal obviously):
http://wiki.flightgear.org/Howto:Protot ... play_StyleI am considering to directlly add a new "Citation" mode to the styles file that can be populated by others - obviously, that will require a new SVG file (compass rose, symbols etc) - but otherwise, developing new ND styles, and refactoring the existing ND code should be fairly straightforward now, because it is easy to directly test all sorts of features without having to load different aircraft.
And in fact, depending on how this unfolds (and spare time permitting obviously), the whole thing could be turned into some kind of "NavDisplay style editor" or even some integrated wizard to easily create/update styles without even having to use a text editor, which is one of the benefits of the declarative *.styles approach, because that can be easily converted to become XML or some other format that can be easily supported using a front-end like this.