I've just been looking on my favourite website and came across this:
Click for more information.Another alternative might be to fit one of these on the end of a short valve:
But even with a short valve, it might still require a short spacer tube on the valve stem inside the rim to stop the valve from being pushed out too far when the tube is inflated.
The 16" rim has the valve hole drilled at an angle on one side or the rim section to allow the valve to clear the side of the hub:
Perhaps you can use a round file to elongate the hole to allow the valve to be angled sufficiently to clear the hub:
Alan