I posted this brief writeup in another thread but it makes more sense here now, so I’m copying it. Hope that’s okay.
First, here’s the threaded section. Its a 12.25mm opening:
Now the interesting bits. Here’s the whole switch cover assembly turned upside-down:
This is made up of two parts. From the outside you’d see them as the outside part (the “sleeve”) and the inside part (the “piston”). Most of what you see in this photo is part of the sleeve, but that bottom rubber ring (top in this photo; its upside-down) is attached to the piston.
The piston just pushes out of the sleeve. Its friction fit and held by the translucent ring mentioned earlier along with the bottom section of that black rubber seal. Pushing the piston out, here’s what it looks like:
There’s that black rubber seal assembly that provides some springyness and water resistance. That leaves the sleeve part left. The translucent nylon bit is attached to the sleeve and it looks like this:
I’m sure that nylon ring could be removed with a bit of persuasion but I didn’t try. Don’t really want to booger it up.
Now for the proof of concept - I reassembled the switch cover assembly and put it back into the tailcap. I did not put the switch back in since any sort of lighted assembly would be on/above that board anyway. Then I held a small flashlight up inside the tailcap and turned it on. The result looks like this:
Now I think that looks really cool! Its kinda orange; not sure if that’s reflections from the red host or if its the tint of the nylon seal, but it works well on this red light. I’ve got blue and green hosts I can experiment with to see how those colors react, too.
Here’s the bad news: that’s 85 lumens I’m shining up in there. 
I could still see the light pretty well down around 20 lumens, but as should be expected, that black rubber seal doesn’t lend itself to light transmission. I don’t even have any clear/translucent switch caps to play with here but it may be possible to cut the black rubber off of the bottom of the piston and use a clear switch cap in its place; I’m not sure yet. But that should require a lot less light to get through there. I’m going to keep poking around at this though because I’d love for my S2+ lights to have this feature.