Success! I've managed to get the drive working again, and I've decided to post instructions for anyone who ends up with the same problem in the future. Please be aware: this, for all intents and purposes, sounds like
the single most dangerous solution you could possibly imagine, and I have no doubt it is.
Please do not try this procedure unless you are at the point of actually getting on eBay to find replacement drives, as I have no doubt this procedure
could do real, definate harm.
Also, this is only tested on a tray-loading iMac with the lens stuck against the outer edge of the tray. If your lens is stuck elsewhere, this might be far less successful (and possibly far more harmful).
Materials: iMac 233 Bondi, Rev. A (though, presumably, this should work on any tray-loading iMac CRT, as they all use the same drive); one paper clip, unfolded (restart button-style);
Procedure:1.) Turn off the computer and unplug the power cord from the back.
2.) Using the unfolded paper clip, press the emergency CD ejector on the CD drive. The CD tray should pop open.
3.) Insert the paper clip into the small hole on the right hand side of the lens carrier.
4.) Try pulling the lens forward (toward the spindle) slowly and carefully. If the lens is in the furthest position back, this procedure will unlock it and allow it to resume normal functionality.
5.) If the lens is, however, not in the furthest position back (it can be pulled even further away from the spindle), continue pulling the lens with the paper clip until it reaches the midpoint between the spindle and the outer edge of the tray.
6.) Close the CD drive.
7.) Plug in the power cord and start the computer. Those familiar with early Mac CD drives (the Performa 630CD comes to mind for me) will recognise the whir of the motor adjusting the lens in the CD drive. This is an excellent sign.
8.) Test the drive. The best test would be to put in a CD that came with the original iMac, such as the System Restore CD. The CD drive should mount the CD and open it (there is no need to install any software; this is just to test the drive). Try this procedure with non-Macintosh format discs as well (i.e. Audio CDs, etc.). If one of these discs will not load, you most likely have a CD-related extension disabled.
Notes and Conclusion:While not related, this procedure also shows how to lock the CD drive (use a paper clip to pull the lens all the way to the outer edge until a rather-loud click is heard and the lens will no longer move.
In my case, the lens was not locked, just stuck.
To determine if the lens is locked in place, put in a CD. If the drive makes sounds and brings up an initialize disk dialog, it's stuck. If the drive is almost silent and doesn't do anything, it's locked.
Thanks for all your help, everybody. As for me, I'm off to upgrade the RAM (384MB in two low-profile memory sticks seems to be the magic number) and install Panther! Wish me luck!