Redirection to GNU screen attached serial port

I'm using GNU screen as a terminal to connect with a usb serial port. eg. screen /dev/tty.usbserial . This works well enough for manual processes. Is there a way to redirect the output of a script running in another session to the stdin of this virtual terminal? ie. I want to repeatedly send some data out over this serial port.
I'm running screen on OS X Leopard.
Thanks
Geoffrey

Does:

$ ./scriptname 2>&1 >> /dev/tty.usbserial

not work?

This would not redirect the input to the script though.