Is there any way for me to use the original Mac Plus or Mac 128k mouse with a modern Macbook Pro? I gather the original Macs used a serial port for the mouse with a RS-422 DE-9 connector. Totally nonstandard today. But surely, at some point some company (Apple?) must have made an adapter to use the early mice with USB-based Macs?

Thanks for any info...
As far as I know, the old mac mice used a DE-9 connector, but this isn't RS-232-compatible. If it was, you could have used an adaptor from RS-232 to PS/2 and then a standard ps/2-usb-converter (both must be active converters).

If you get an ADB-mouse you will be able to use them as there are adaptors for these.

If you're into electronics and programming, you could build your own converter eg. using a teensy-board for few bucks. It shouldn't be too hard as the old apple mouse just sends quadrature signals which should easily be translated into pointer movements.