Probably only able to do this on the ol' fat boy consoles, and even then it'll be a chore as you really don't want to locate that stuff over the heat sink. Looking at my v9 while it was open, I only saw one kind of good spot to put it, and that was the HDD expansion bay... it would fit in there even if the user was using it (granted, it'd be a tight fit and not so sure it would work with the 'official' PS2 HDD, probably just regular IDE ones).
Be interested to see pictures of this when it was done (hell, I'm tempted to attempt it myself

)