I’m pretty sure cv is initialized as a global var within opencv.js. When I swap my built opencv.js for the stock downloaded one, the setup I posted works as is. Maybe the built opencv.js doesn’t have a defined cv var?
EDIT: According to this, “once opencv.js is ready, you can access OpenCV objects and functions through cv object.”
So, I’m not quite sure 
According to this, calling cv should work without explicit initialization, which is why I’m leaning towards thinking I’m doing something wrong in the actual building process somehow.