When you kill another player a cargo crate should drop with a trophy
(player) kill trophy

so for example if I die a cargo crate with a Drevent kill trophy should drop

people would be able to collect them like rare objects (not that mine would be rare)

Well sounds like collecting ears in diablo. Not sure this is a good thing for peace and be nice on #100.
Why not instead a widget for the victim saying your first death was offered to you by XXXX?
It would be available at your last home station for your eyes only.