Plutonium

@Hobo-Tim
Look at the decompiled : \maps\mp\zombies_zm_pers_upgrades_functions.gsc
In bo2 there's a perma perk that gives you the ability to keep perks even after you get downed. (but idk the name)
I think the function is pers_upgrade_perk_lose_restore
You could try to look there in my opinion and adapt the function to your project.