The way I address the lack of Burning Embers support in classicons.lua

[Aerials]

I doubt this is anything close to perfect, but it works (step 1).
All I did was modify the code of classicons.lua slightly... It's as a separate file / element though, as it didn't seem to work otherwise.

Is there something that I missed in oUF that makes this unnecessary?

embers.lua

--[[ Element: Embers
 Toggles the visibility of icons depending on the player's class and
 specialization.
 
 Widget
 
 Embers - An array consisting of five UI Textures.
 
 Notes
 Warlock - Embers
 
 Examples
 
   local Embers = {}
   for index = 1, 5 do
      local Icon = self:CreateTexture(nil, 'BACKGROUND')
   
      -- Position and size.
      Icon:SetSize(16, 16)
      Icon:SetPoint('TOPLEFT', self, 'BOTTOMLEFT', index * Icon:GetWidth(), 0)
   
      Embers[index] = Icon
   end
   
   -- Register with oUF
   self.Embers = Embers
]]
 
local parent, ns = ...
local oUF = ns.oUF
 
local PlayerClass = select(2, UnitClass'player')
 
-- Holds the class specific stuff.
local ClassPowerType, ClassPowerTypes
local ClassPowerEnable, ClassPowerDisable
local RequireSpec, RequireSpell
 
local UpdateTexture = function(element)
    local red, green, blue, desaturated
    if(PlayerClass == 'WARLOCK') then
        red, green, blue = 1, .49, .05
        desaturated = false
    end
 
    for i=1, 5 do
        if(element[i].SetDesaturated) then
            element[i]:SetDesaturated(desaturated)
        end
 
        element[i]:SetVertexColor(red, green, blue)
    end
end
 
local ToggleVehicle = function(self, state)
    local element = self.Embers
    for i=1, 5 do
        element[i]:Hide()
    end
 
    (element.UpdateTexture or UpdateTexture) (element)
 
    if(state) then
        ClassPowerDisable(self)
    else
        ClassPowerEnable(self)
    end
end
 
local Update = function(self, event, unit, powerType)
    local element = self.Embers
    local hasVehicle = UnitHasVehicleUI('player')
    if(element.__inVehicle ~= hasVehicle) then
        element.__inVehicle = hasVehicle
        ToggleVehicle(self, hasVehicle)
 
        -- Continue the update if we left a vehicle.
        if(hasVehicle) then return end
    end
 
    if((unit and unit ~= 'player') or (powerType and not ClassPowerTypes[powerType])) then
        return
    end
 
    if(element.PreUpdate) then
        element:PreUpdate()
    end
 
    local cur = UnitPower('player', ClassPowerType)
    local max = UnitPowerMax('player', ClassPowerType)
 
    for i=1, max do
        if(i <= cur) then
            element[i]:Show()
        else
            element[i]:Hide()
        end
    end
 
    local oldMax = element.__max
    if(max ~= element.__max) then
        if(max < element.__max) then
            for i=max + 1, element.__max do
                element[i]:Hide()
            end
        end
 
        element.__max = max
    end
 
    if(element.PostUpdate) then
        return element:PostUpdate(cur, max, oldMax ~= max)
    end
end
 
local Path = function(self, ...)
    return (self.Embers.Override or Update) (self, ...)
end
 
local Visibility = function(self, event, unit)
    local element = self.Embers
    if((RequireSpell and not IsPlayerSpell(RequireSpell))) then
        for i=1, 5 do
            element[i]:Hide()
        end
        ClassPowerDisable(self)
    else
        ClassPowerEnable(self)
        return Path(self, 'UpdateVisibility')
    end
end
 
local ForceUpdate = function(element)
    return Path(element.__owner, 'ForceUpdate', element.__owner.unit)
end
 
do
    if(PlayerClass == 'WARLOCK') then
        ClassPowerType = SPELL_POWER_BURNING_EMBERS
        ClassPowerTypes = {
            BURNING_EMBERS = true,
        }
        RequireSpell = WARLOCK_BURNING_EMBERS
        ClassPowerEnable = function(self)
            local element = self.Embers
 
            self:RegisterEvent('UNIT_DISPLAYPOWER', Update)
            self:RegisterEvent('UNIT_POWER_FREQUENT', Update)
        end
 
        ClassPowerDisable = function(self)
            self:UnregisterEvent('UNIT_DISPLAYPOWER', Update)
            self:UnregisterEvent('UNIT_POWER_FREQUENT', Update)
        end
    end
end
 
local Enable = function(self, unit)
    local element = self.Embers
    if(not element) then return end
 
    element.__owner = self
    element.__max = 0
    element.ForceUpdate = ForceUpdate
 
    if(ClassPowerEnable) then
        if(PlayerClass == 'WARLOCK') then
            self:RegisterEvent('SPELLS_CHANGED', Visibility, true)
        end
        ClassPowerEnable(self)
 
        for i=1, 5 do
            local icon = element[i]
            if(icon:IsObjectType'Texture' and not icon:GetTexture()) then
                icon:SetTexCoord(0.45703125, 0.60546875, 0.44531250, 0.73437500)
                icon:SetTexture([[Interface\PlayerFrame\Priest-ShadowUI]])
            end
        end
 
        (element.UpdateTexture or UpdateTexture) (element)
 
        return true
    end
end
 
local Disable = function(self)
    local element = self.Embers
    if(not element) then return end
 
    self:UnregisterEvent('SPELLS_CHANGED', Visibility)
    self:UnregisterEvent('PLAYER_TALENT_UPDATE', Visibility)
    ClassPowerDisable(self)
end
 
oUF:AddElement('Embers', Update, Enable, Disable)

then in oUF.xml, add:

<Script file='elements\embers.lua' />