How do I test restful APIs with constantly changing and random user defined data

I'm developing a clean-up API (http://ift.tt/1Onexkf) for a Mediawiki application, Baka-Tsuki to pull meaningful data from the novel project pages like author, volume lists and cover images. The pages are user-defined and formatted in various ways decided by the translator(user). The pages are also updated and created daily, with the creation of new formats occurring sporadically. However, the API parser is able to handle most, if not all of the current pages, no matter their format.

Baka-Tsuki is not going to change into a database-based application in the near future, since the wiki is currently the most user friendly and cost-effective way to share translations, and we don't have enough developers to constantly work on a new application.

I'm looking into using mocha to automate testing of the API but as the input data constantly changes, testing is nearly impossible without checking every page available. I've looked at twitter and facebook testing methods but they have constantly formatted user input.

Is this case, which testing method should I refer to? Should I run the test simply based on the types returned, and the availability of the values returned or do I have to make a copy test-page to stimulate testing?