Hi,
Like Shantala mentioned earlier, BOM is stored as a structured product in CRM and the plant has no significance in the system. The filtering based on the plant is done on CRM side in the inbound function module COM_MAP_BAPIMTCS_TO_PROD_IL. Sub-routine process_bommst handles the logic to check the plant from the table crmm_pr_mast.
Creating a group BOM would be of some help but I am not sure how this would affect any process in ERP system.
Hope it helps,
~Athreya