Seeing as nginx requests can be done by their URI, you could have a /write prefix to anything that handles user input, which wouldn't be cached? Especially on sites that don't have masses of user writing (in which case you'd use some sort of intermediary queueing daemon that nginx could shove things to, and could slowly feed the things into your backend).

Writing the /write prefix would immediately invalidate the cache for the changed pages and thus they could be easily regenerated a single time and placed into the cache until the next time (or perhaps the pages could simply be invalidated once every 10 seconds or so - very little has to be truly realtime).