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.

But replies from the mailing-list are not integrated into the pull-request comments, so it's not a 1<=>1 integration.

The problem with this is in turn that perhaps the pull requester is not subscribed at all in the ML...