After attaching to the process I saw that the PID was writing to the error log after I sent the USR2 signal (d'oh! should have checked there!). In the error log I have the following lines (one for each time I attempted to send `kill -USR2` to the master PID):
2014/06/04 15:45:04 [crit] 1018#0: the changing binary signal is ignored: you should shutdown or terminate before either old or new binary's process
2014/06/04 15:45:04 [crit] 1018#0: the changing binary signal is ignored: you should shutdown or terminate before either old or new binary's process