Oh, I should mention - given that deltamods that add a rune also add the empty slot boost, like so:

Code
new deltamod "Boost_Weapon_Rune_LOOT_Rune_Venom_Large"
param "BoostType" "Rune"
param "MinLevel" "10"
param "MaxLevel" "14"
param "Frequency" "1"
param "ModifierType" "Weapon"
new boost "_Boost_Weapon_EmptyRuneSlot",1
new boost "LOOT_Rune_Venom_Large",1

It's reasonable to expect that the system requires an empty slot before a rune can be added, so having a query for retrieving the number of slots on an item would be useful. Something like this:
Code
query ItemGetRuneSlots([in](ITEMGUID)_Item, [out](INTEGER)_Slots)

Then we could check if an item has any rune slots at all before attempting to add a rune.

I sort of attempt this with ItemGetRuneItemTemplate, but that both fails if there's nothing in a slot, or if an item doesn't have a slot at all, so unfortunately it's not ideal for seeing if an item has an open slot available.

LaughingLeader's DOS2 Mods