Is there a hidden config variable for the "Word Separator for URL Titles" setting? What other hidden config variables remain undocumented in the ExpressionEngine User Guide?
Yes, there is
$config['word_separator'] = "dash";
$config['word_separator'] = "underscore";
Devot:ee has a page dedicated to this: http://devot-ee.com/ee-config-vars
You can also see any hidden config values that add-ons might have if you turn on the Output Profiler and then on the front end of the site you click Show on "Config Variables" which will show all the config items and their values.