An alternative to my suggestion, based on the fact that Stephen’s plugin uses wp_mail, which itself uses PhpMailer.
I’ve now found that it’s possible to set a callback function in PhpMailer. Its ‘action_function’ property can be used to set the name of a function to be called after the email is sent.
I still feel that the ‘pta_member_contact_message_sent’ hook is in slightly the wrong place, but for now at least there’s another way to send an acknowledgement to the submitter.