Credit to Jake Robinson for this tip, which automatically creates a BOM for new items:

if record.categ_id.id == 6:
      env['mrp.bom'].browse(1).copy({'product_tmpl_id': record.id})  

Notes:

1. The “apply on” means that it will not create a BOM for service items.
2. Selection by product category is done in the Python code.
   • You need to know the record IDs of the product category and the BOM you will be copying (in this case the product category id is 6 and the BOM we are copying is record 1).
3. You could easily have different BOM templates for different product categories:

if record.categ_id.id == 6:
     env['mrp.bom'].browse(1).copy({'product_tmpl_id': record.id}) 
if record.categ_id.id == 7:
     env['mrp.bom'].browse(2).copy({'product_tmpl_id': record.id})