When remotely enrolling a student, RPC should pass to the remote server all flags affecting the student's privacy (maildisplay) and how Moodle send mails (emailstop, mailformat, maildigest).
These settings affect the user in the course even before his first log-in, when all user data are passed.

I attach the two modified functions in enrol/mnet/enrol.php
(modifications commented with [nicus] tag)

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function enrol_user($user, $courseid) {
global $MNET, $MNET_REMOTE_CLIENT;

$userrecord = get_record('user','username',addslashes($user['username']), 'mnethostid',$MNET_REMOTE_CLIENT->id);

if ($userrecord == false) {
$userrecord = new stdClass();
$userrecord->username = addslashes($user['username']);
$userrecord->email = addslashes($user['email']);
$userrecord->firstname = addslashes($user['firstname']);
$userrecord->lastname = addslashes($user['lastname']);

// [nicus] added emailstop, mailformat, maildigest, maildisplay, lang, trackforums fields
$userrecord->emailstop = addslashes($user['emailstop']);
$userrecord->mailformat = addslashes($user['mailformat']);
$userrecord->maildigest = addslashes($user['maildigest']);
$userrecord->maildisplay = addslashes($user['maildisplay']);
$userrecord->lang = addslashes($user['lang']);
$userrecord->trackforums = addslashes($user['trackforums']);

$userrecord->mnethostid = $MNET_REMOTE_CLIENT->id;

if ($userrecord->id = insert_record('user', $userrecord))

{ $userrecord = get_record('user','id', $userrecord->id); }

else

{ // TODO: Error out return false; }

}

if (! $course = get_record('course', 'id', $courseid) )

{ // TODO: Error out return false; }

$courses = $this->available_courses();

if (!empty($courses[$courseid])) {
if (enrol_into_course($course, $userrecord, 'mnet'))

{ return true; }

}
return false;
}

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function req_enrol_user($userid, $courseid) {
global $CFG;
global $USER;
global $MNET;
require_once $CFG->dirroot . '/mnet/xmlrpc/client.php';

// Prepare a basic user record
// in case the remote host doesn't have it
// [nicus] added emailstop, mailformat, maildigest, maildisplay, lang, trackforums fieldS
$user = get_record('user', 'id', $userid, '','','','', 'username, email, firstname, lastname, emailstop, mailformat, maildigest, maildisplay, lang, trackforums');
$user = (array)$user;

$course = get_record('mnet_enrol_course', 'id', $courseid);

// get the Service Provider info
$mnet_sp = new mnet_peer();
$mnet_sp->set_id($course->hostid);

// set up the RPC request
$mnetrequest = new mnet_xmlrpc_client();
$mnetrequest->set_method('enrol/mnet/enrol.php/enrol_user');
$mnetrequest->add_param($user);
$mnetrequest->add_param($course->remoteid);

// Thunderbirds are go! Do RPC call and store response
if ($mnetrequest->send($mnet_sp) === true) {
if ($mnetrequest->response == true) {
// now store it in the mnet_enrol_assignments table
$assignment = new StdClass;
$assignment->userid = $userid;
$assignment->hostid = $course->hostid;
$assignment->courseid = $course->id;
$assignment->enroltype = 'mnet';
// TODO: other fields
if (insert_record('mnet_enrol_assignments', $assignment))

{ return true; }

}
}

return false;
}