I assume you installed the drivers for the adapter in OSX?
In sheepshaver, have you tried disabling appletalk, or to set it to use the ethernet (even if it doens't exist)
It seems there are also many drivers for usb-serial adapters that can be installed in OS 9. I don't know if they are helpfull in any way, but if they do something for the serial port software in OS 9, then perhaps you get lucky.
I found OS 9 drivers for e.g., FTDI chips http://www.ftdichip.com/Drivers/VCP/Mac ... E2.sit.hqx
also some here: http://serialio.com/download/Drivers/USB_RS232_OS9.zip
Is this of any use? http://www.planet-rcs.de/article/mac_serial_port/