I went to a local electronics shop and picked up a few plugs hoping I could get them to work. One of them was a close match (6 pins), but the shapes didn't line up. The next one I found was almost a perfect match, but there's only half the pins, and it fits a bit loose. I'm going to try to glue two of them together and dremel a bit of the edges off so it'll fit, but I need to work out how to get it to clip to the plug.
Here's a picture of the plug I snapped with the webcam...
If that doesn't work, I guess it's time to clip the old Amiga power supply....