This is a really interesting idea and, and I like it. I’ve been thinking about something sort of along these lines, but instead of a spring I was imagining a small soft silicone sphere or something. 
It hadn’t even occurred to me to try a spring - I think there are a lot more possibilities to tune the exact feeling that way.
I bet there are springs out there already that are just about the right size to fit in that channel and interface with those stems that have a sort of stepped bottom to the rod.
All that said - I also think a similar feeling might be possible with a combination of progressive / complex curve springs and silicone dampeners on the stem. Not too long ago I found a frankenswitch combination mixing progressive springs with U4 stems that gives a nice clean break leading into a soft, cushy bottom-out. It might not feel like this mod would, but I’d at least use similar words to describe the feel.
Edit: @HungerMechanic I’ve been doing lots of experimenting along the theme of dampened clicky / crisp tactile with a soft landing lately; you might try something like this frankenswitch that mixes BOX Royal and Silent BOX Brown components.