Try adding
"$RCMAIL->output->command("plugin.showNotification", array(
'subject' => "It's a test...",
'from' => "Peter",
'uid' => "10"
));"
after "$RCMAIL = ..." in show_notification() in html5_notifier.php
I don't know how RC manage it, but I read that the RECENT flag doesn't work correct with some IMAP Clients. I could implement a notification showing "You have X new messages" instead of the messages. This sould work, because RC has a special function for this.
Regards,
Tilman