If bounties were set on empires rather than avatars/units, it would maybe be simpler. Then you could set a bounty on an empire and other empires would be able to collect parts of it for all the damage they inflict on the target empire. That is how it worked in Sins of a Solar Empire, and it was pretty cool. Pirates would act like normal, but if they happen to inflict damage on your empire they get some of the bounty too.
In short, set bounty on target empire. Other empires attack the target empire and get bounty reward for inflicting damage to the empire, such as getting the wroth of a spacecraft in bounty when you blow up the ship. Destroying buildings could give bounties depending on the volume of the building destroyed. And capturing worlds could give bounty depending on the total amount of volume captured.
In short, set bounty on target empire. Other empires attack the target empire and get bounty reward for inflicting damage to the empire, such as getting the wroth of a spacecraft in bounty when you blow up the ship. Destroying buildings could give bounties depending on the volume of the building destroyed. And capturing worlds could give bounty depending on the total amount of volume captured.