I believe it would be nice if fulcrum would have ldap as an option for auth.
To be honest this is fairly unlikely to be implemented any time soon. I may eventually look into other auth strategies, and it would certainly be technically possible, but it's not a high priority at the moment.
Perhaps you could borrow the auth code from gitlab. it supports plugins.
Or maybe use http://wiki.phys.ethz.ch/readme/devise_with_ldap_for_authentication_in_rails_3