John - great that you are thinking of ways to extend the life of our mighty Minis. In this case the part in question is not really needed for functionality, even with an original style SU fuel pump that has an external breather facility. They will operate whether the tube is connected or not. My feeling is that the original style breather fitting is really only wanted by those doing a sympathetic restoration or a concours car where an authentic part is mandatory. Your description suggests a nylon nut will be visible when viewed from inside under the back seat.
I certainly don't want to throw cold water on your suggestion but you asked if there is any interest. I'm aware that another member has a good original bung and is looking at getting them re-manufactured, perhaps using the 3D printing technology. If I needed one I'd go that way.
But that's just my view.
