Hi Akhila
BPM is not really necessary for this. Using two async scenarios (JDBC to IDoc, and proxy to JDBC) as mentioned in your approach is fine.
This was a common scenario in one organization I was at before. Instead of using an exit in the IDoc posting step, the posting details was logged into a custom table. There was a periodic background job that queries the custom table and triggers an outbound interface back to PI, which is routed to a JDBC receiver with the posting details.
Either approach is fine and it really depends if there are any preference in your organization on usage of custom table or user exit.
Rgds
Eng Swee