$0.02 from someone who has been in CQB- armed and unarmed- many times. Unless both combatants are very well trained, any close-quarters melee or unarmed fight will inevitably devolve into grappling. At that point, the man with the knife underhand or the free elbow will be the only one to get any decent blows in. The objective when grappling- unless you excel at a style that uses close-in grappling such as Pancratean, Krav Maga or some more advanced forms of Judo or Aikido- is to break free of the entanglement to re-engage with strikes. If you don't have the room, either because of conditions or you cannot break entirely free from the grapple, elbow and knee strikes come into play. If your elbow is above the line of your opponent's shoulders (such as when he is wrapped around your chest or waist), striking the neck and temple with the elbow is bloody brutal, and unless you have no power behind it, can really bugger an opponent up. Even more so if he has you around the waist and you can rotate to cross-strike (rotate around to the left so that your right elbow strikes his right temple, including the force of unwinding the twist). Knees come into play in some instances, but hitting someone in the groin is not as disabling as movies and books make it out to be. Been there >.<
As for ad-hoc weapons, the more the merrier. Hitting someone with a fist, knee or elbow does damage to them, but it also does damage to you, albeit much less. Getting something to take that abuse for you is always a welcome enhancement. I looked at a few pictures of guitar slides, and they don't seem to add enough mass to increase the damage of a punch, nor do they cover and protect the knuckles. However, for a hammer-fist strike to the face or temple, they'd do some right nasty damage as mentioned before, concentrating the blow into a ring-shaped area. If you want alternatives, never underestimate the nastiness of a set of keys clenched between knuckles and pointing outward between the fingers, almost like miniature punch daggers, ESPECIALLY when aimed at the neck or eyes.