Can you braze? I haven't done this before but I believe it would work. Using a needle for inflating balls, cut off and then braze the hole shut for the needle? An old boss said you have to know when you are screwing elephants and when you are making watches. This would be the latter. That is my first thought on an easy fix.
Edit - you may also be able to epoxy an extremely small screw into the hole for the needle portion shut from the inside of the thread in area ??? and cut the needle off ??? I say a screw because you would want something with a head on it so the pressure doesn't blow it through.
Keep in mind you can always drill & tap it to a known thread and put a real screw/plug in it. Again - the latter above
