Some of you have said some of what I want to say, but none of you have said all of it, so I apologize for resaying some things. Just take it to mean I agree.
Let's break it down.
1) There are melee weapons that use the blade skill, the clubs skill, and the unarmed skill. The first two have an accuracy, the last uses the physical limit.
2) The power of Critical Strike works with a particular melee skill. It will work with a sword if chosen for the blades skill, a stun baton if the clubs skill is chosen, and there is likewise no question that it will work with a stun glove, knocks, et all. For that matter, it probably even works on elemental body. If you've chosen it for unarmed, and you're rolling unarmed dice, it works. It would work with cyber spurs! Critical strike (blades) wouldn't.
3) Killing Blow, Elemental Strike (which is useless) and Penetrating Strike all say they work with an unarmed attack. It is not clear to me if this means literally a naked fist, or just any attack that uses the unarmed skill. It could just be effectively stating a more specific version of the limitation on Critical Strike. If anyone knows of an applicable errata or FAQ, please let us know.
4) It is clear that you could have weapon foci knucks. (or a shock hand, for that matter). I also feel it is fairly clear you could have weapon foci tattoos, these presumably would enhance your bare fist attacks. What would be weird is if you stacked a shock glove or knucks on top of that. I dunno what happens then. But I think it's clear you could use penetrating strike and elemental strike with the weapon foci. You'd need to use killing hands if you wanted to do physical damage, for that matter.
Those are the only facts I feel we have.
My personal feeling is that knucks are an extension of your bare hand attack in a way that shock gloves and such are not. They're just making your knuckles a little harder. I've been using Killing hands with Knucks without really thinking about it, my GM raised no objections. I'm pretty sure that if I did so with my shock gloves, objections would be raised. Punching someone with your hands on fire wearing densiplast gloves seems fine, burning someone while using a shock hand (or burnign and shocking?!?) seems weird and cheesy and probably destroys most people's sense of immersion in the game. (even though fire dmg really doesn't do much). Adding the -4 AP from penetrating strike on top of the -5 from shock gloves seems like cheese city.
There is no special rules justification for this, it is just my feeling on what is "reasonable." It all comes down to the definition of what an "unarmed attack" is, which as far as I know, is not well-defined.
You can get a much better effect than knucks using bone lacing or reinforcement, and penetrating strike would definitely, no question, work with that.