The regex in config.js doesn't match my configuration.php layout (Joomla 2.5) - it uses var instead of public. The fix is simple - just remove the 'public' prefix and leave it simply as '$'. Here's the altered initialisation for config_file in config.js (sorry, didn't have the energy to do a proper diff :) :
var config_file = fs.readFileSync(config + '/configuration.php', 'UTF-8'),
match,
re = /\$(\S+) = '(\S+)';/g;