Not sure how that's supposed to work. An included module is always an ancestor, so there AFAIK is no way to call a method of the including module using super.

Instead of the polluting alias I could use a local variable like this:

    orig_active_admin_form_for = instance_method :active_admin_form_for

    define_method :active_admin_form_for do |resource, options = {}, &block|
      options[:signed] = true
      orig_active_admin_form_for.bind(self).call resource, options, &block
    end

Sadly, this causes a segfault (using Ruby 2.1) when SignedForm::ActionView::FormHelper.form_for calls super and I don't know why.