I actually saw this recently on
another forum. Someone posted a change to ItemRack that lets it distinguish between Evocation and TLC. The only catch is you have to mod it yourself
In the ItemRack folder, open localization.lua and replace line 283 with this:
["script"] = "local evoc=arg1[\"Interface\\\\Icons\\\\Spell_Nature_Purge\"]\nlocal evocSpell = GetPlayerBuffName(\"Evocation\")\nif evocSpell and evoc and not IR_EVOC then\n EquipSet() IR_EVOC=1\nelseif not evocSpell and IR_EVOC then\n LoadSet() IR_EVOC=nil\nend\n--[[Equips a set to wear while channeling Evocation.]]"
Basically it checks whether you really have the evocation buff, and not just a spell with the same icon.
Disclaimer: I don't have a lightning capacitor (my Karazhan raid is drowning in melee drops), but others have reported that this change does work.