Could that particular idol be simply gold leafed? That would account for the sort of blotchiness of the surface...
They used a dulling spray to prevent glare from the lights and possibly seeing the camera and crew in the reflections. The same as they did for C-3PO.
Over the years I've seen many production idols and one thing I think that has been finally put to rest is the theory that the eyes had holes drilled in them as we see on so many repros.
As far as the ones with the real eyes, I think one was used for the pedestal that showed the eyes obviosuly not plated, the one we see with the eyes moving back and forth on screen as Indy approaches. Since that one had obvious pupils they needed consistancy so they had to add pupils to the rest. A real idol would not have glass eyes and since the 3 second scene it was used for was a wide shot where you couldn't really see the eyes too well, it worked just fine. However, I'd think that any other scenes would have the eyes covered in gold.
What I think they did was take several castings of the same idol Brandon has and plated one, interting the eyes in later, and then did one or more with the eyes, but plated it after the eyes were inserted and so we see the shape of the surface of the eye as it would be on a real eye. The cornea is slightly raised from the rest of the eye and I think this is what other idols eyes looked like. They were shaped like real eyes just as ancient sculptures used to look like. They would sculpt in the cornea and maybe even a small circle for the pupil. I believe they then painted the pupil area in as we see in the pic of Ford above and when it's held up in the air before the Hovitos. Remember, this painted eye version was only on screen for a matter of seconds so I'm sure they weren't too concerned over how the paint would look since it was flashed by so quickly. After all, much of the Star Wars stuff was poorly painted but looked fabulous on screen.
When I look at the enhanced pic above of it being held up by Belloq I see gold on the eyes. I can't see the glass off the eye in any of the frames or photos I've looked at. I only see gold on the eyes.
I did this pic below in a quick minute. Just thought it would be fun to see how it would look in one piece.