Would a slow request logging middleware configured to be a bit longer than the 
nginx timeout work? A quick search turned up dogslow. 
http://pypi.python.org/pypi/dogslow

Right now, users sometimes hit HTTP 504 Gateway Timeout on pages that take too
long to load.

The administrators don't have an easy way to detect this. This ticket will track
that problem until it is fixed. (Suggestions welcome.)

The architecture, fwiw, is that nginx is a front-end to Apache. nginx would
probably store 504 in its log.