Well, the times have changed. It’s no longer really acceptable (or legal, for that matter) to just add people to a mailing list anymore, without getting a double opt in from the person via a link they click on in their email. It’s become impractical as well, since email senders will not want your business either if you continually cause their server IPs to be blacklisted.
This certainly makes it difficult for chasing down people who don’t like to click links in their email. I guess if you are small and you know everyone already, you can always find workarounds, but the software can’t support this type of usage because it will be abused.
It sounds like it’ll be hard given your clientele, but I think you’ll be better off tackling this problem at the other end - find ways to get the people to sign up and opt in, rather than trying to hack the software.