Aha! Apple's Universal CD ROM Driver 5.3.1 from Mac Driver Museum does the trick under System 7.5! It doesn't seem to work under 7.1, though. Their notes say it was released with System 7.6, so maybe 7.1 is too much older for the extension to work correctly. That's good enough for me to declare success: works on one machine, half-works on the other machine.