They probably have a matrix code for the actual values corresponding to armor permutations, then use an encryption matrix to get the code you see in the URL. That way changing one value in the code will affect multiple values for the other armor permutation matrix. In other words, there's almost no way to figure it out.
Have a thread that might help? Bungie.net : Halo: Reach Forum : The Halo Reach Armory - UPDATE *Spoilers*
Sorry, but that thread isn't much help at all. What we were hoping was that each image's URL would be a combination of codes which would match up with colours and armour pieces, similar to Halo 3's. Going through and saving each and every possible combination and reuploading for the armour generator just wouldn't be possible because it would take far too long (I'm talking months or years of non-stop work) and the resulting database of images would take up several terrabytes of space (thats 1000's of gigabytes)
There's actually a guy that uploaded a youtube video of all the armor permutations. I think it's because he played locally that he had access to all of them. Either that or he modded them, but if he could take screen shots and then get them on his computer... that would go a long way in creating one.