Here are a couple diagrams showing a possible method of deployment. This one was second in my mind to the Parachute Tube, so I only made a quick mock up of this one to check it out. The main problem I see with it is the nose cone is not very rigid when it is cut in half, but I think this could be worked around with some reinforcing, or possible a support in the center that would keep the g-forces from squashing the nosecone down too hard. Click the underlined links below each drawing for a closer look.
Clamshell closed for launch