I have once built a DMX controller with an Arudino clone (Teensy) and the tricky part was the timing.
Since the cloud communication does need some time, this might be an issue here, but have you seen these two threads?
http://community.spark.io/t/dmxserial-port-to-spark-core/3151
http://community.spark.io/t/spark-core-dmx-shield/4200
Maybe you could tag the guys there, they’re are awesome and most likely willing to help 