Ok so far, for testing, I created a "version 1" the of nexus zone, as well as a few versions of the regular spawns in there, since some actually have regular loot items on them. (I used Percy_Clutches to test). I put a script on an npc in the pok, simply using "plugin::SendToInstance" and after I killed Percy, I was able to loot the robe and staff, no problem.
After that, I created another version of snpool, but with a custom NPC and fully custom item on it. Again, I used an npc script to enter the instance with:
Code:
plugin::SendToInstance("solo", "snpool", 1, 137, -5, -19, "EXP", 360);
I was able to loot the custom item without issues, but I am not sure just exactly what your setup involves at all, maybe something a little deeper ? (I used "solo", rather than group or public, just for testing)
Edit - Forgot to add, I used a quest::addloot under a simple EVENT_SPAWN script in the snpool instance with the custom item and I was still able to loot it no problem.