I had to check the variables with my test group. The result is that there is no simple method to do this.
A Special Notice is from the Owner but the reply is to the group. The From line structure is:
[Group Name]@groups.io on behalf of [Owner or Moderator] via Groups.io <[Group Name]+owner@...>
The only current way to do this would be to visit your Members page under Admin, scroll all the way to the bottom so that all members show on the page (assuming you still have the default Infinite Scroll in your preferences), then go back to the top, check the checkbox next to Display Name and then choose Action -> Send Message.
That will definitely send the message to each individual with the From line showing:
Even so, I'm not sure why you would choose to do this rather than simply posting to the group. Depending on your member count, you would invite a potential onslaught of messages that could last for weeks.