There should be a rule that stipulates if your character is carrying a corpse or an unconcious player they should have to emote it. Another alternative is to have a PC flashing a certain color to symbolize that they are holding a corpse. Another alternative is to not allow BO mode during instances of mass PVP without special DM permission. [Since obviously a person walking near death can be seen. and would require some degree of roleplay to move the person.]
Literal corpse flag.
Stick a distinct flag/banner on someone's back if they're carrying a body, and allow the examine player tool to get the names of any corpse(s) that person is carrying.