I'm not sure that there is any well supported way to do this, but if you really really need to hack it you could probably modify sys.path at the top of your python script.
When you do ". /opt/ros/hydro/setup.bash" the PYTHONPATH environment variable gets set to "PYTHONPATH=/opt/ros/hydro/lib/python2.7/dist-packages". The opencv python module is there, in "/opt/ros/hydro/lib/python2.7/dist-packages/cv.py".
When your python script starts, it sets up its module search path (sys.path) using the PYTHONPATH environment variable. You can modify this and it will change where python looks for modules:
#!/usr/bin/env python
import sys
sys.path = ['/path/to/your/opencv'] + sys.path
print sys.path
# do stuff....
This will cause python to see your installed opencv3.0 before the opencv that got installed by ROS. This is a pretty big hack though, so it might not go completely smoothly.