Well, if you have any of your old wigs, maybe you can spray a bit of grey hair paint into them (comb it afterwards, to get it to streak like the hair is greying).
If I recall correctly, in Man In The Iron Mask, he was still in the Kings Musketeers, their leader, so he would still be wearing their coat of arms color, or a uniform of some sort. What do you have that can be put together as such? Not knowing exactly what you have on hand to work with makes it a bit harder to know. I would think a sash of some sort, they are French after all, and loved to gussy up, and show your rank.
Pantaloons if you have them, or dark breaches if you don't. Do you have time to grow some facial hair? The traditional mustache and gotee? If not, see what you can find for that, and again I would hit it up a bit with some grey hair paint, or some baby power to grey it up a bit.
Of course a huge hat with poofy feathers is a must.
Leather gloves, or even white long armed gloves (French again).
Bows, big fancy bows on the tops of your shoes, on the sash, etc.
Tall droopy boots!
Ascot, or ruffles!
Here is something I found from Wikipedia:
http://en.wikipedia.org/wiki/Musketeers_of_the_GuardLike the first guy in the line up. Poofy!