Right now (0.9.8.50) it is not possible to subscribe to slot changes for entities because there is no script invoker and no way to mod the EntitySlotInfo class to forward EntitySlotInfo::OnAttachedEntity and EntitySlotInfo::OnDetachedEntity to other components etc. The newly added parent change events on script component unfortunetly fire before inventory or slot info states are processed, so they provide 0 information about "how" the parent attaches the child (raw AddChild vs Inventory storage slot vs slotmanager)

I would propose adding a similar event handler like InventoryItemComponent::OnParentSlotChanged either on the manager with old slot new slow, or two separate invokers for added and removed on the individual slots that an outside component can subscribe to via looping through SlotManagerComponent::GetSlotInfos.