See this thread in the zeromq-dev list. It appears that the 0MQ source makes extensive use of the POSIX API, so some porting to an OS without POSIX is required. In addition, this requires a TCP / IP stack, which may have a larger size, which itself is 0MQ.
FreeRTOS - , API POSIX RTOS , 0MQ ( ).
RTOS API- POSIX POSIX. RTOS NuttX . API- POSIX API BSD.