Looks really cool! I'd love to see an open grammar definition format for all kinds of tools, both command-line and GUI.

I've worked on parsing TrueType files, and they have some really "interesting" grammars. For example, they have lookup tables that define offsets at the beginning of the file. Also, a format flag at the beginning of a table might define the structure of the rest of the table.

It seems that once you decide you want to be able to parse all of this, you're grammar will turn into a Turing-complete language. Have you considered this? Where do you stop?