I'm just saying it's very confusing and unfriendly especially for new players. I've seen twice in the last week or so I've been playing again where a new player has died in a instance and thought they lost their untradable items because when they went back to Zul-Andra or where ever it may have been to reclaim their items the untradables were't there.
I'm not sure how you keep Perdu useful that's on you to figure out but how it's currently setup in mine and I'm sure many others opinion it's bad qol.
edit: Maybe for Perdu you can give him option to "enchant" untradable items for a hefty price and you can keep them on death when you die inside the wilderness. If you die in the wilderness with a untradable it gets turned into broken version similar to how OSRS does it. Then you can use the item on Perdu to repair them. https://oldschool.runescape.wiki/w/Death#Breakable_Items
edit2: You can have perdu sell this item https://oldschool.runescape.wiki/w/Trouver_parchment and use it with what I suggested above.