I've configured:

• 1 backend added to a product with a path (i.e. "/backendpath")
• no mapping rule at the product level
• "catch-all" mapping rule "/" at the backend level (i.e., prepended with the backend path in the proxy config)

So 'curl url/backendpath/' work and 'curl url/backendpath' doesn't work because of missing slash at the end. But it should work.