Let me see...
On the normal texture: Try a 40 to 60% black on the alpha part to the glittery bit. (trial and error - or experience and feel). Paint the colour slightly darker than the final colour you want on the finished fuselage. Don't necessarily add the glitter (metallic flake) to the base paint.
On the specular: paint the glittery bit the same base colour but slightly more intense. i.e. if the glittery bit is cherry red then make the specular part a slightly brighter red. Then using a very vine spray brush, with a big particle spread, spray the "glittery" sparkles over the colour (make this a separate layer/object so you can copy it). Use maybe "gold" or "silver" (ochre or grey).
On the specular alpha - use a very light alpha but copy the glitter layer ("glittery" sparkles above) and paste over in exactly the same position. Darken this almost to black (90%ish). Offset this loyer/object 1 pixel "north and west"
Copy this glitter layer over to the normal texture's alpha layer and offset it 1 pixel "north and east" and make sure that the dots are almost 100% black (start with 90%)
The more pixel resolution you have, the better. 2048 texture sheets and big maps will look a lot better.
If you can create the spray pattern for the glittery bits as fine and small as possible, then that is good too.
Experiment and have fun!