I'm not sure about whether it's possible to rewrite a CD-RW that is in use, as in the Tails case. However, since it is trivial to re-write a CD-RW while it is not in use (e.g. after it's unmounted during the shutdown process) that's a more likely avenue of attack. An attacker with full access to a running Tails system could rewrite our memory erasure script so that it loads the necessary CD-burning software into RAM on shutdown and then re-writes the CD-RW once it's unmounted.

As I understand it, the TAILS Live CD on a CD-RW gets mounted Read-Only when booted up, so it is not possible to rewrite it while in use. I know this because I only use CD-RW CDROMS with regard to this question. Otherwise, I use USB flash drives, and it too is mounted Read-Only.

While mounting as read-only (and the use of the iso9660 filesystem, with no multisession) certainly complicates things, there might be no absolute guarantees that they cannot be modified as long as the hardware allows it, which both CD-RW and USB does. It may be the case that it's just some part of the kernel that prevents remounting them as read-write, and that part can be changed by an attacker with full access to the system.

It's still a very theoretical attack and nothing I'd be too worried about, especially since I'm only conjecturing (see "might" and "may" above). In fact, I'd be really happy if someone could prove that my conjecture is wrong.