Had the same problem at a cruise in a while back. Used the method mentioned to open the hood with a long screw driver up through the grill. My hood release cable is short and I can only get the retainer on the very end of the cable.
To avoid this problem in the future I drilled a small hole in the latch and installed a screw to hold a thin cable. I then installed a small Eye bolt in the side of the vertical support for the latch assembly. I then threaded the cable through the eye bolt and down thru the opening in the splash pan at the radiator drain plug. Cut a small wooden dowel and drilled a hole to attach the cable for a handle. I can now just pull the handle to open the hood.
Now I have an alternative method to open the hood should the retainer come off again.
Was an easy, simple, cheap and fast fix for the problem.
David Mac