First of all, welcome to this forum
I must say, I have some problems with this argumentation, too.
1) I can't see why a Sea Dragon Cloak should not be armor. It does nothing else than providing armour saves.
2) Further, slayers would never wear any protection whatsoever. they seek a heroic death on the battlefield.
3) As sartori said, special equipment of warbands can only be bought by those warbands themselves. The only way for others to get access to it would be by capturing a warrior carrying the desired equipment (a Dark Elf warrior goes out of Action by a warrior of that warband and rolls "captured" on the Serious Injury Chart afterwards).
4) the rules for troll slayers state in the rules for "Dwarf Treasure Hunters", p. 22:
"Troll Slayers may be equipped with weapons chosen from the Dwarf Warrior equipment list. Slayers may never carry or use missile weapons or any form of armour."
To protect the slayer you could give him a pair of dwarven axes and give him "master of blades". It lets him parry two attacks if he matches the to hit roll instead of surpassing it. And he can even reroll these parry attempts.
Further, dwarves are generally very powerful when it comes to defense, as they can raise to Toughness 5 and are only out of Action on a roll of 6.
But you can even improve that with dwarven skills: True Grit and Thick Skull. Even if he cannot wear a helmet, avoid being stunned on a 3+ is still more than worthwile.
And if you want to make him even harder to wound, you can also give him "resilience".
and for a little protection that also works against shooting, you could give him a lucky charm. And if you're really rich - an elven cloak. but again, there might be some fluff restrictions against this, too...