Mark,
That should work in .qmail.
To get it to work for certain users take a look at /etc/e-smith/templates-user/.qmail/15sortspam
You will need to add a database property for each user, say TMDA, and enable or disable.
You can pretty much copy 15sortspam but make the necessary changes to use a different database property and a different output.
Or you could spend $90 and get the TMDA package from Stephen Noble at dungog.net. This will provide you with a Server Manager panel to enable or disable TMDA per user.
Have you taken into account the fact that confirmation emails will be blocked by the goodrcptto plugin of qpsmtpd. It will not like the tagged reply email address. The respondents may have to use the confirmation link in the challenge email (if you have tmda-cgi installed).
Personally I hate TMDA and refuse to respond to the challenge emails. If someone doesn't want my email the first time I am not going to go out of my way to make sure they get it. I know a lot of people who feel the same way. I also know a lot of people that started using Vanquish and stopped using it because no-one was responding.
Jon