mart256:
I think there is another option. You have usb host on the yun, right? Connect an ftdi breakout to it and you'll get an extra serial port on the linux enviroment.
Very nice idea.
Plan C:
USB to UART bridge:
FTDI USB Quad Serial Converter(FT4232H), 4 UART ports
FTDI USB Double Serial Converter(FT2232D, FT2232H), 2 UART ports
Openwrt has driver for both IC.