I don't know what a signature tag request is but it sounds like groups.io may not be a good fit for these offer announcements. Even email is not a good mechanism as there is no way to ensure that all recipients receive the message at the same time, if that's what's intended. I know of no way to have the message emailed but not displayed on the web site. Even locked topics are still visible and the member can then check their email to reply directly to the poster.
A group text message with the email of the poster and info on the offer might be a better approach as that is more "real time".
Just a thought.