I have a few ideas for you. I'm new to leatherwork myself, but I've also been studying mold making and in particular how to make plaster casts for making leather masks. I think you are headed in the right direction. What I would do at this point is make a plaster waste mold of the torso form. Here's the basic process: http://www.peterfors...cess_waste.html. You can get a big bag of plaster relatively cheap at Lowe's/Home Depot or maybe through a ceramics store. You might be able to put a thin layer of clay over your figure form and then sculpt the armor in clay on top of the form. Then you can apply the plaster to make the waste mold. Remove the waste mold once dry, and make a plaster cast of the torso form "wearing" the armor. Then stretch a lightweight wet veg tan leather over the armor portion (somehow tacking or fastening it so that it's very tight). Using tools you can force the leather into the shape of the armor beneath it. Keep working the leather until it dries and it will keep its shape. I got this method from a cool book about making masks (http://www.amazon.co...k/dp/1558701664) in leather. I lucked into a cheap used copy, but you might try to find it through your library if you don't want to buy it. This is an Italian mask making techique that uses a cow horn hammer (not available at Lowe's haha) and small wooden tool called a sticketta to force the leather to take the shape and detail of the plaster beneath. Here is a page where a person used this technique to make a mask, but I'm sure you can imagine how it could be used in your project: http://www.wastekeep...bumName=album35. I would imagine doing this process twice (for front and back of the armor) and any other shoulder pieces, etc., trimming up the leather pieces and then sewing or fastening them together.
A different technique that somebody already mentioned is using silicone based products to cast the armor from the form. It would probably be more expensive, but this is how Hollywood would do it (and maybe the people who created the costume you are replicating). Check out the Batgirl costume on this page: http://www.smooth-on...p?galleryid=416. They might have cast the armor in neoprene which has different formulations from rigid to semi-rigid to flexible. It is easily painted, lightweight, and very durable.
At any rate, good luck on your project! Keep us posted on your progress!