Hi,
I suggest you set external breakpoint at program LBBP_WS_APIF11.
This program is to fill product detail.
In fact, from catalog, we can't retrieve product guid, but only product id.
But within this program, it will help to determine the logical system for product source system, and correspondly read out the product from comm_product table as long as you maintain this product.
However if you would like to automatically determine the source system, I am afraid that you have to use bbp_transfer_catalog or bbp_doc_change_badi to realize this.
BR,
Ivy