Have the boomerang sprite have an instance variable "Parent" in which, on creation, you store the Base.UID
Ex:
=> On start of layout
--> System: create Base somewhere on layout
--> System: create Boomerang on Base.x, Base.Y
--> Boomerang: Pin to Base
--> Boomerang: Set Parent to Base.UID
With a Boomerang pinned to the base you only need to bother about Base's trajectory, the Boomerang with a rotate behavior will rotate on itself directly.
And you can have a
Base: On destroyed
Boomerang: Parent = Base.UID
--> Boomerang: destroy
Picking the Boomerang instance that has the same UID as the currently picked Base does the job.
Same goes to pick Base by UID => Boomerang.Parent (if you want to pick the Base related to an already picked Boomerang)
Example commented capx