The ProVari 510
threads and top cap are one-piece steel and the centre pin is pretty much jammed in with a delrin pressure washer. It would need major milling skills to try and fit a spring loaded pin. I can't see how you could do it with the limited space available.
That's the beauty of the ProV, it's solid. It also means that adapting it would be difficult. In my opinion the ProVari would benefit from a spring loaded pin.
If you manage it then post a picture. I would be up for it if it was relatively simple.