US citizen presently in Phnom Penh requesting to visit Yunnan province for less than 30 days.

Stephen, you can probably apply for it from there. Just have a try.
If you apply for a tourist L type, first thing is to prepare the required materials: your passport and copy of its data page, the completed application form, a passport-style photo, your valid visa for Cambodia, and proof of transportation and accommodation arrangements. And then submit the application to the embassy of PRC to apply for it.