No sources cited.

Warriors of Kerensky doesn't list any of these in its rules for zellbrigen. The same is true for FM: Crusader Clans, FM: Warden Clans, and Invading Clans.

  1. Firing at the rear of a 'Mech is prohibited, unless the opponent exposes it during combat.
  2. Firing at a fallen 'Mech is prohibited. Opponents must be allowed to regain their footing before combat resumes.
  3. Firing at an immobile or shutdown 'Mech before the initiation of combat is prohibited. A 'Mech which has shut down due to excessive heat may be fired upon.
  4. Firing at a MechWarrior that has ejected from his or her 'Mech is prohibited.
Indeed never saw such rules. For the ejected MechWarrior, Clan Honor dissuades MechWarrior ejecting from their 'Mechs, so the question is solutionned !
The rear firing means, IMO, it is unClan-like to first shoot an ennemy from the back, meaning before the duel, to engage him. Otherwise, during the honor duel, rear firing is permitted.
I would also add
  • Intentionally moving out of the line of sight of the opponent is prohibited.
I never saw this rules too, as some Elementals won over 'Mechs in Trials using their small side to surprise their foes.--FIVE-one 13:30, 15 May 2009 (PDT)

The whole article looks a bit strange to me. I would narrow it down:

  • Each warrior will issue a challenge to a different enemy. If one side outnumbers the other, then the extra warriors on that side will stand aside until one of their comrades falls in battle. However, they are free to fight units not deemed worthy a personal duel (e.g. tanks, conventional infantry).
  • No units and weapon systems may be used, that might attack several enemies at a time. This inludes for example artillery and minefields.
  • Melee combat is only allowed as a last resort.
  • A warrior may not shoot a retreating foe or one that has been granted hegira. This includes ejected pilots.
  • Firing at a shutdown 'Mech is prohibited, except when the Mech shut down due to the opponents excessive build of heat.
  • Firing at an immoblie Mech is a matter of both duelists personal honor. The immobile opponent should know, when it is more honorable to give up, than to waste a Mech.
  • Intentionally staying out of the line of sight of the opponent is prohibited. --Detlef 04:42, 16 May 2009 (PDT)
I would nuanciate the first point : tanks are sometimes used in trials. Many Clans consider those units unworthy, but others don't. For example a Crusader Clan, Clan Hell's Horse, doesn't think the same way. So I don't think we can generalise those unit status as being unworthy targets. I know Clan Field Manuals states armor and infantry (except Elementals, often following zellbrigen rules) not subject to those rules but we've got to keep this in mind.
I don't know why would be artillery cited in zellbrigen rules, as this is not a dueling weapon. It's only a matter of warrior to warrior duel. Artillery is a dezgra weapon for many Clans. For artillery mounting 'Mech, like the Naga, I never read anything about the interdiction of using its artillery weaponry against a sole target in a duel, if it only target its dueling foe.
The immobilized 'mech is, as you pointed out, a MechWarrior consideration. From my experience, its often solved by hegira.
At last, I still don't know why staying out of line of sight intentionaly is unClanlike, as some Clan Warriors use hit-and-run tactics even in Trials (for example I don't think Lincoln Osis won his Trial by staying in line of sight of his enemy's 'Mech crosshair). No Sourcebook states that rule. --FIVE-one 13:24, 16 May 2009 (PDT)