Location:
phpbb/notification/method/messenger_base.php, notify_using_messenger(),
inside the loop over the queue, just before the messenger's assign_vars()
call that merges USERNAME and U_NOTIFICATION_SETTINGS with
$notification->get_email_template_variables().

The same spot exists on 3.3.x and master (on master it is inside the loop
over the messenger methods).

Purpose:
Let an extension add or change the template variables of a core
notification email, per notification and per recipient, before the email
is rendered.

Use case:
Companion to core.messenger_template_paths (PHPBB-17722 for master,
PHPBB-17750 for 3.3), which lets an extension supply its own copy of a
core notification email template. That copy can only use the variables
the notification type already assigns, so it cannot show anything new.

The request behind both: add the poster's email address, and a link to ban
its email domain, to the post_in_queue email sent to moderators. Today the
only ways are to replace the whole notification type service, or to rewrite
the finished text in core.notification_message_email - string surgery on
already-rendered text, with no placeholder and no per-language file.

With this event, a listener checks the notification type, looks up the
data it needs from the notification's item_id (the post id for
post_in_queue), and adds its own variables, which its own template copy
then uses.

Variables:

• notification (\phpbb\notification\type\type_interface, read-only): the
    notification being sent; get_type(), item_id, item_parent_id and
    user_id identify it
• user (array, read-only): the recipient's user row
• template_vars (array, modifiable): the variables about to be assigned
    to the email template: USERNAME, U_NOTIFICATION_SETTINGS and everything
    from get_email_template_variables()

Proposed name:
core.notification_method_messenger_modify_template_vars