I built something similar (not quite as pretty) for parsing SWF tags in Ruby: http://github.com/daeken/Arienette/blob/master/swfTags.rb

Also in that repo is the beginning of a Ruby compiler for Flash, as part of an implementation that was intended to compile Flash apps to native iPhone apps, before Adobe announced they were doing the same thing. Might be of use to someone.

PADS is a related (and more ambitious) line of work from some folks at AT&T: http://www.padsproj.org/index.html

What are the advantages of this over converting the specification into a CFG and then using a parser-generator?

None, because that's what he's doing.