When you submit a pull request, github sends an email to the addresses you have on record. All that would be required for your suggestion would be to add the project's mailing-list to the list of emails on the account. Though emails would be sent for every other project on your account also.
The problem with this is in turn that perhaps the pull requester is not subscribed at all in the ML...
But replies from the mailing-list are not integrated into the pull-request comments, so it's not a 1<=>1 integration.