Is there a Quantcast stat equivalent to the metric ‘Unique Visitors’ or ‘Absolute Unique Visitors’ that my javascript-based web analytics package uses?

Yes, you should compare Unique Visitors/Absolute Unique Visitors with the ‘Cookies’ statistic that we provide for Quantified Publishers.

JavaScript-based analytics packages calculate a site’s Unique Visitor number by counting the number of unique cookies seen over a specific period of time.