It is my understanding that nginx can query memcached for data that it serves, but you have to place them there using some other (homegrown) program. So the question is, can I have nginx fetch data from the backend and place them into memcached for future use for certain locations? Sort of not having to place all files from /images in it "by hand" but having them (mem)cached after being fetched for the first time?
Any backend other than memcached would suffice also.
As the page you linked to states, the backend (your application) must put the data into memcached.