I recently tried Kaitai for something like this. It worked for some nontrivial cases, but I felt like I kept getting bitten by syntax limitations. All the way through, I kept wondering if writing a one off utility might be faster. But it was possible to express variable structures, sizes and counts. I was using the syntax and the cmd line visualizer, but might have been more productive with their html/gui tooling.

http://kaitai.io/