There should be a way to set items already placed in a container upon its creation. (While this might technically fall under #61, I still feel it would be easier to have this as a dedicated feature)

Editor could be similar to how the loot effect is edited, being able to interact with each slot in the container editor to decide what item goes in each slot.
Things to be mindful of : you can change the size of the container (need to manage the interaction with the item slots on size change), container can be set to only contain inner items (filter item browser for inner items only if it is checked, what if a non-inner item is in a slot when we check that...)

Did the onCreate method ever get exposed for use? I've got a (belt) container that I'd like to populate with several (vial) items.

This issue hasn't been worked on yet.