The new fuses arrived today and my first reaction was "Ooh, they're teeny small!", but I have a stereo microscope with 20x magnification. So, off with the old fuse and on with the new one. The original was 0603 after all, but the 0402 fitted OK. And the DS started up just fine! It goes straight into a set-up dialog, so it may have forgotten some config, but it runs and charges OK.
Suitably encouraged by my success, I remembered that incident with the dead iRiver MP3 player. Time to get down to it and replace that Philips UDA1380TT I2C DAC chip. The chip implements most of the audio part of the MP3 player, including DAC, line out, headphone out and the ADC for line in. And I'd blown up one channel of it by connecting it to a suspect audio amplifier.
A few weeks back, I got a new chip from RS Components, but I hadn't fitted it yet. Back to the stereo microscope with a sharp knife to cut the old chip out, then solder the new one in. It's a package with 0.65mm pitch leads. Then, reassemble the device -- I took it apart back in November last year. Success! The battery needs to be charged, but the headphone output works again! So I now have a 40Gb MP3 player, from FreeCycle, that can record audio as well as play back. It'll also run the alternative firmware, RockBox.
I got two UDA1380 chips, so I have another one to solder to a break-out board and wire up to the Arduino for fun. Next thing to fix will be the ultrasonic cleaning tank that I was given by someone at Dorkbot. I have already acquired two BU406 power transistors for that.