So I finally got the arm mold open. :\
In pieces.
Several.
Basically what happened is that the additive I used in the plaster bonded with the acrylic clearcoat that I used to seal the clay. I know how to avoid it in the future though, and that's what matters.
As I was looking at the mold I realized that I may be able to salvage it yet. Noticing that the upper arm was still mostly intact, I suddenly recalled a certain episode where Gluttony encounters Scar in the library.
That's right bitches. BATTLE DAMAGE GLUTTONY IS IN THE HIZZY. I've already opened the right arm with minor damage, so it's totally usable. I'll have a near perfect right arm, but my left is going to be severed at the elbow. I'm going to try and patch the whole arm mold if I can, I will have two full arms. But for now I'm happy with BATTLE DAMAGE GLUTTONY. :)
Here's some more pics of the mold
I had a good foam run last night and made two great casts of the hands (of which I'll only be using one though). So now all that remains is the arms, sewing the costume (if my vinyl ever arrives, which is another story entirely), painting the foam, and the general detailing of the costume. Who knows, I may even make it yet.