What do you think to the idea of adding a button to allow players to open the backpack of down players?
This would mean players wouldn't have to kill another player to open their backpack?

I am open to the idea in general, but I don't think this feature belongs in this particular plugin because this plugin was intended to decouple some of the convenience and cosmetic concerns from the core plugin, not actually add new capabilities. For instance, being able to open your own backpack was already a function of core Backpacks, even without the built-in GUI, since you can use a command to do it. Being able to open a downed player's backpack is a completely new capability. If I were to implement this feature, I would do it in the core Backpacks plugin or as yet another addon plugin (one whose purpose was to add a new capability).

Regardless of which plugin this functionality is added to, some minor changes would be required in Backpacks to make it possible.

Some technical details will need to be figured out, including how we will do distance checking (when you loot a container, your loot panel automatically closes when you move away from the object, but Backpack looting doesn't have that concept today), plus compatibility with plugins that do some tricks for looting players such as Inventory Viewer and Bag of Holding.

Thanks for the detailed answer and explanation around some of the technical difficulties much appreciated.
It's always nice to gain a better understanding of how things work from the otherside. Thank you WhiteThunder.