Hello to whoever might be listening. I'll try to detail my situation and maybe someone will have insight into why this isn't working.
I have the following:
USB power adapter from the Chromecast box
Prefab OTG cable
Homebrew OTG cable
USB stick imaged on OSX
USB stick imaged on Windows 7
Soldered a couple male USB connectors and a female USB connector to rig these items together in various configurations.
While holding down the Chromecast button, the best I get is a couple blinks out of the USB sticks. It seems like something is happening, but it doesn't kick off the entire imaging process.
Both OTG cables work to connect a mouse to my Nexus 7.