I have a Pentel Orenz Nero 0.2mm with such a feature. It is pretty nice, but you will often scratch over the paper with the metal which is definitely a counter argument. Makes 0.2mm usable though. I think for 0.5mm you don't need it, just get some decent 'lead'.

I have a Pentel (0.5) that I purchased 12 years ago. I treat it with equal ritual and reverence as I do, say, my Korean crossbow, or Leica optics - it has its own altar of sorts. I may have appendages of my own form that are guarded less. I modified it though, using a glue to permanently fix the pocket clip into a particular position.

I was hesitant to post a trivial comment, but I discovered an ancient container of 0.5 pencil inserts this morning, and... I worship high quality Pentels. Built to last!

There's a "pipe slide" model of the kuro toga that will slide the pipe with the writing so only a little bit of lead is exposed at all times.

Uniball also has custom lead with a softer outside and a harder core.