Heh, I just came here to see if anyone had done something similar. I just stumbled across a Japanese page where the guy had done something similar for his 6300. My 5400 had a dead battery when I got it so I bought one from ebay to replace it. Then I left it idle for too long and that one also died. So it's been left dead ever since. I never thought about using 3xAAAs as a replacement.

BTW I was told some time ago that it's possible to remove cells from a 9v battery to make it into a 4.5v. I just did a quick google, and found this.