Some guy over at Macintouch has written in with a ridiculously simple workaround to gain access to the disabled RW features of newer superdrives. Due to the crappy layout of Macintouch and lack of a direct link to the relevant passage, I'm reposting his note here:

If you put a fresh DVD-RW in your Superdrive, it spits it back out. However, get a friend with a DVD-RW on a PC to write something onto it. This will mount on your Mac just fine. And guess what? Toast will now erase it and from now on it can be used like any DVD-RW! It's as killer workaround that I have tested with no problems.
He's doing this on a UJ-825.

His full note is currently at http://www.macintouch.com/newsrecent.shtml but beware, yesterday it was at http://www.macintouch.com/index.shtml#i.2004.09.07.super