Yeah man, I'm in a hardcore band and we've got a shot at going commercial! Anyway...
To really do a good growl, you want to hyperventilate yourself first. Breathe out 3x, each outward breathe being at least 3-5 seconds long. You will feel light headed...don't worry. Then, breathe in as much as you can and let out the growl. It helps if you tuck your chin into your neck, as it allows for the saliva to mix in with your vibrating epiglottis. And, just do as you said with the whole diaphragm thing. If your
using your throat at all, your doing something wrong.