After learning of the ! print portnames command from John, I finally got my PowerBook’s LogoChip Desktop Software talking over my USB serial to my LogoChip.
Turns out the Classic mode support and the Java serial communications driver installation error were red herrings. The problem was simply that LogoChip Desktop was defaulting to the first serial driver in the list–which was the IrDA driver. Since my desktop computer doesn’t have IrDA, it picked the USB driver–the only one in the list.
The solution was simply to update my config.txt to read:
com-port: /dev/tty.usbserial0
I had previously tried changing it to usbserial0, but it didn’t recognize that format. It was the ! print portnames command that led me to the device-file syntax.
And it works. Hooray!
Detailed instructions on the class wiki.