Gist of our rails_admin mountable engine config solution

post_config.rb

module RailsAdmin
  module PostConfig

    def self.extended(base)
      base.class_eval do
        rails_admin do
          weight -10

          list(&base.rails_admin_list_fields)
          edit(&base.rails_admin_edit_fields)
          edit(&base.rails_admin_edit_extra_fields)
        end
      end
    end

    def rails_admin_list_fields
      proc do
        sort_by :published_at
    
        field :title  
      end
    end
    
    def rails_admin_extra_fields
      proc { }
    end
  end
end

post_config_decorator.rb

RailsAdmin::PostConfig.module_eval do

  def rails_admin_edit_extra_fields
    proc do
      field :new_field
    end
  end

end