You can add functionality into you process to make an "add CC action" available in your process; See page 135 of the LDSD Administrator guide for the steps to setup.
Once you've implemented this it would be reasonably straightforward to spot these guest raised incidents via a process condition and branch off through an automatic action that adds a CC recipient as the email address captured on the main form. Then whenever the raise user is notified by email the CC recipient (real user will also get a copy).
That's given us some direction. We do have some 'Add CC admin' functionality created within our implementation already so we're looking at replicating that in the relevant processes. So it seems to be that once you add a CC address to a particular ticket in a process, all notifications from that ticket will copy in the CC address for the life of the ticket. At least we're hoping that's what happens. I'll update this as we progress.