I'm part of a team of developers for Impact (http://impact.sf.net).
First of all, let me congratulate you on the simplicity of grammatica. I had a look at JavaCC but was soon lost in the examples - mixing grammar and code. grammatica seems much cleaner.
We need to extend Impact with more interfaces. First step will be a parser for IGES graphical format (http://www.nist.gov/iges/)
This format consists of four main blocks (Start, Global, Directory, and Property) where some have fixed formats and some use separators.
I'm concidering using grammatica as the basis for all future Impact parsers. Will it be able to cope with advanced file structures such as IGES?