Hello,
It is simple.
BRM-Implement a BRM using the below link-
in BPM-
Import the mapping which is having the BRM call.This will fetch values from BRM and will be passed in BPM.
In PI-
Trigger messages to BPM.BPM will call BRM using mapping and output will be generated.
I have completed this scenario,Soon will post a document on the same.