I've come up with two more links for us. This site explains how to move the battery contacts from an old battery onto the PSG-650 battery (the plate based one). It also has pictures of the inside of the Apple battery, which shows the internal cells visibly. This site seems to be selling a new battery with a case and contacts compatible with the Macintosh Portable.
As I don't have any battery right now, I can't tell how easy the Apple battery would be to open, put new cells in, and close up again. Buying a new third party battery is pretty attractive, as it would probably cost me nearly as much to buy a dead Apple battery (but they usually come with an additional Macintosh Portable as a bonus item).