application_mailer.rb

class ApplicationMailer < ActionMailer::Base
  def self.add_templates(template_names, block = -> {})
    template_names.each do |key|
      define_method(key) do
      end
    end
    define_method(:template_name, &block)
  end
end

export_mailer.rb

class ExportMailer < ApplicationMailer
  add_templates Export.destinations.keys, -> {
    # @export is nil,
    # as this seems to be called from the class, not the instance of ExportMailer
    @export.type
  }

  def send_export(export_id)
    @export = Export.find(export_id)
    @user = @export.user
    mail(to: @user.email, subject: subject, template_name: template_name)
  end
end