(Are there any attorneys on HN familiar with, for instance, the rights to privacy of the deceased? I'm pretty sure they don't have any.)
I think it's best just to file a civil suit rather than trying to set up a system that the Supremes could very well deem unconstitutional.
What might work is for municipalities to write into police labor agreements that XX% share of any civil judgement precipitated by your actions will be borne by you alone. You're still jointly and separately liable, but the municipality would be able to go after the officer in a separate action.
Something along those lines would accomplish the same thing without bringing constitutional questions into it.
American cops already seem to murder with impunity, it’s hard to see how more accountability could possibly make them worse.