How do I find or request a copy of someone's thesis if this person did their work in mainland China?
I am currently in China, as a postdoc. From my experience with the inner workings of Chinese academia and culture, I must say your objective is not an easy one. Most of my Chinese colleagues will only move by either by personal interest or when receiving an order. This thus is how you should approach this situation.

The proposed strategy of contacting a dean or some other local authority is a valid attempt, but you should make sure that person would be interested in helping you (by transmitting an order) for some reason. Otherwise I'd locate the respective advisor stating that you're interested in obtaining a thesis because [enter here something which will benefit that person, citations/publication/funding/collaboration/etc]. Try calling them directly while asking their name first-thing, which should put them temporarily in a responsive position.

Finally the last resort is to show up in person, where you will be treated like an authority while visiting. 

The Chinese are quick to ignore communication on topics which they have no interest, and there is little casual chitchat meaning that the fact that you've written to some people about your request is unlikely to be mentioned to others. 

Finally concerning availability and language, my bet is that the mentioned thesis is in Chinese with an English abstract, sitting in a cabinet full of others' theses in an office of the department. If you manage to get someone to help you, the easiest would be that a student takes pictures on a phone of every page and sends by Wechat or QQ.

Good luck!