Anything lengthy I might write here would only be a summary of what’s on the link I provided (and the section immediately above that, about creating custom auth objects), so I’ll let them speak for themselves. The specifics to your situation would be to use
class BackendFormAuthenticate extends FormAuthenticate { }
with literally nothing inside the { }. So all you’re doing really is creating an alias for that class. (It still seems like there should be a better way to do so, but I’m unconvinced that my ideas about it would work given that you need to provide a namespace and class name to the Auth configurator.)
Hope that helps.