until somebody develops a solution an easy way to have a messenger welcome message:

• Assign a support profile, this profile will be sending the welcome message and will continue to provide support to the users.
• Add a field to sys_accounts table like "welcomemessagesent" default to 0.
• create a php (or your prefered language) code to scan the UNA database for new accounts with email/phone confirmed and welcomemessagesent=0
  with your code for each new user:
• add a new record to messenger lots table
  id: leave empty
  title:leave empty
  url: leave empty
  type:2
  created: unix time stamp
  author: your support profile id
  participants: the new accounts profile id","support profile id
  class:custom
  
• For your message to the user add a new record to messenger jots table
  lot_id: from the previous table
  message: your nice message
  created: unix time stamp
  user_id: your support profile id
  new_for: new users profile id
  edit_by:0
  last_edit:0
  from the systemaccounts table make welcomemessagesent :1

put your code to a cron job.
This algorithm will welcome new users and provide them a support account that they could contact for future needs.
For a crowded UNA site it is better to trigger the welcome message as soon as the phone/email is confirmed instead of a cron job.

Ps: if you don't want your current users to get this message set them to welcomemessagesent 1 before the first cron job.