Oh, right, model not mold...hmm...Sean Bradley said:Close.
The clay original is called a model, not a mold. Molds are the rubber or plaster negatives that are use to make copies.
But essentially, yes this is how moldmaking works for sculpted armor.
To answer the preivious questions though, I think you should go with a brushable resin in order to create a hollow, wearable casting. You don't want to cast solid and hollow it out later... you'd be throwing your monet away in material costs.
Yes It has been done before... this is how scultped armor is made...
The rubber you use to cast this will not stick to the clay, but if you are concerned, you should buy some spray on mold release. Use that stuff and nothin will stick to nothin.
Also what type of clay are you using there? You have to be positively certain that your clay does not contain sulphur. Sulphur inhibits the rubber from curing, so you might end up with a big sticky mess if you just use any ol' clay.