I finally got my hands on the Brown/Aluminium Putty by P3. It is indeed like Greenstuff: all you need to do is kneading the two components together until they are mixed up evenly (and the putty gets a brown-metallic look) and then you are good to go.
The difference to Greenstuff is noticeable right away. The Brownstuff is firmer and not as sticky, this way you can start even with some more finer modeling right away. I sculpted the carapace shape with the pluggers and it stayed... more or less. The Brownstuff has a "memory effect", but it is not as drastic as with Greenstuff, and the readjustments are easy to do. What is also easy (and fun) to do with the Brownstuff: corrosion.
Even though this was more or less a test I am very happy with the result, and assume that the next segment is going to be even crisper.
I am not sure if I can replicate this very specific look of the original carapace 100%, but I am confident that it will be close. Once the Blight Drake is painted and all the rust and weather effects are applied it will look consistent for sure.