Well I'm with you on that, I've never heard of anyone making or using an external focusing ring for an arctic S3 before either.
Practically speaking, it doesn't make sense to do IMO because of the how the chasis of this laser is constructed and how the bezel completes the module. But that doesn't mean the 'disc' focus ring that comes with the arctic underneath the bezel couldn't be replicated and replaced with something that had an extended 'neck' that protruded through the minimized inner diameter of the stock bezel/headpiece after removing the plastic shield.
Your best bet would be to search some respected memebers here like Mohenberg, Flaminpyro, or Jayrob (to name a few) to see if your idea could be milled out and made possible. That's what this forum is all about :beer: