I made several attempts with SQL triggers, none of them worked adequately, unfortunately.
I am assuming it disrupted part of DNN's registration process. I tried numerous methods to circumvent this, such as adding a delay to inserting the row with the additional RoleID but that made the submission of the registration form hang for the duration of the delay, and then errored out.
At one point I had it working to the point where everything went smoothly EXCEPT when a user completed the registration form, there was no confirmation of completion, it made their account and registered them, but the form looked like it was never submitted; some profile properties were missing too.
Another point I had everything appearing to go smoothly, except, the user didn't get registered to "Registered users" and "Subscribers", which are RoleID's 1 and 2. So I had those hardcoded to add those roles automatically as well. But things would get all haywire and errory when I tried to edit the users profile or add more or less roles.
So, I'm officially frustrated. May Odin show mercy.