It’s not entirely what you want but there is a very excelent Bison mode for Emacs.
http://www.emacswiki.org/emacs/BisonMode
This is a better link:
http://ftp.lip6.fr/pub/emacs/elisp-archive/incoming/bison-mode.el.gz
Since Yacc and bison are so closely related I don’t find it lacking too much. Though I do most of my work with Antlr anymore.