active_admin_boolean_params_pre_process.rb

ActiveAdmin.register SomeModel do
  permit_params :flag

  module ControllerWithParamsMapping
    MAP = { '0' => false, '1' => true }

    def permitted_params
      mapped_params = super
      if mapped_params[:some_model].present?
        mapped_params[:some_model][:flag] = MAP[mapped_params[:some_model][:flag]]
      end
      mapped_params
    end
  end

  controller.prepend(ControllerWithParamsMapping)
end