First a few quotes from RMC Spell Law:
"It was discovered that it was possible to make a direct link or bond with the souls or spirits of certain mortals. This link allowed the immortal to gather the dangerous raw essence, and send it down these channels in far more controlled and manageable flows. This new, weaker but more manageable form of casting, combined with intervention from these powerful beings, meant that more mortals could become casters, and being one became far less dangerous." RMC Spell Law page 12.
"Channeling casters draw their power from their deity, so local fluctuations in the essence field, or even the absence of a local essence field will have no effects on their ability to cast. Metal seems to interfere with their divine connection, so when casting a Channeling spell, the caster must wear no metal armor, ... . More importantly their ability to draw power is based on their relationship with their deity, so their actions and behavior can have serious impact on their ability to draw power or cast." RMC Spell Law page 30.
"Channeling represents the powers of the deities of a given world ... . A spell user of this realm may draw their strength from their respective deity, and often does not require that deity’s cooperation ... . More powerful
and significant spells, however, such as death spells and the revival of the dead, might require the active consent of a deity (depending on the world system)." RMC Spell Law page 89
The Realm of Channeling is one of the most misunderstood of the three (four with Arcane) Realms. I feel most of this confusion comes from the Channeling Companion which drastically altered the canonical understanding of the realm. Markc is right in that the power from the deity is still essence from an outside source, but unaffected by the local flows. It is not the power of a god granted to a mortal but a resonating sympathy between the object of worship, the Cleric (or channeling using subject) and the flows of essence through that sympathy (channel). Metal seems to interfere with that flow of essence similarly to essence users. The channel (sympathy) is a passive aspect between the deity and the channeling users until higher level spells contact or involve the deity to acquiesce, at the very lest, in the ultimate effect.
The Paladin’s efficacy with Transcend Armor is more an occupational hazard than a flaw in the system. To fulfill his role, a Paladin needs this affinity and learns it. The passive nature of the channel means that the deity does not take a direct interest in overcoming the limitations of the realm.
Powers that come directly from the deity might not have those limitations, as they are direct manifestation of divine power and might, and not a passive sympathy (channel). The recipient of these granted abilities may not even have the capability to cast spells (low scores), but is granted special abilities from the deity represented through Talents, Backgrounds or Gifts.
The sympathy between the deity (or subject of worship) and the channeler would be broken when the channeler greatly transgresses against the ethics, morals, ideas and/or goals of the deity. He would lose access to the essence from the deity.
Metal interferes with essence users, channelers and even mentalists when worn on the head. Each of these interferences relates to the source of essence, how it is processed and used. In channeling, it does not deal with the will of the deity. The world system of each gaming group could adjust these accordingly, but as canonically represented within the rules, metal interferes with essence regardless of the source or application.