Doing a full-color HUD with scripting should be possible right now. What isn't available is all the information needed to create a full HUD. Animations, etc. are also possible, but you have to write the scripting to go through each of the frames in the correct time period, using the texture handle properties (FPS, etc) and ba.getFrametime().
I don't know what other people are doing for the HUD. IMHO the only "proper" solution is to rip the old system out and replace it with something new that's capable of reproducing the original. I think that is what someone is doing. Preferably with a dynamic, master list of all the gauges by name, and supporting parent/child relationships between gauges so that you can switch on and off the toplevel gauges, or change their colors, and their children will switch on/off, or scale their colors back accordingly.
That reduces the amount of work to making HUD gauges work for RTT or scripting to coding things in for that one HUD gauge class, and then letting the individual gauges override the draw functions where needed to draw that gauge properly. The draw function would probably take something like a texture index, and then optional UV coords, so you could map each gauge to a texture or multiple gauges to one texture (to minimize rendering time).
One thing to note is that the HUD may be dependent on the viewer. If you want working 3D cockpit gauges you might not want to be locked in to assuming that the viewer object is the one drawing the HUD. Or you might want to draw the radar from another ship for cinematic effect, or maybe for some special mission gimmick (receiving radar or visual feed from a spy ship, for example).
