That's going to be a tough one. Even if you can find a device or setup that allows you to send data through Ethernet to USB, Onyx might not allow you to choose TCP/IP as an option for that model. I have one customer who just sends the jobs through the illustrator plug-in that is connected to the network and then has a laptop RIP by the printers. He send all the jobs he needs over and then has to go into the printer room and send them to the printer. You can set up Rasterlink to automatically process jobs that show up in the list. So the best answer I can give you is:
Have a laptop connected to the printer with Rasterlink installed, make sure the hot folder is set to share, setup the hot folder in FineCut for illustrator to be the networked hot folder, and then set Rasterlink to automatically process jobs. You will want to set up multiple hot folders for different profiles and settings so that you can just send the job to the folder that has the settings you want to use.
I get this question once in a while and always wonder what the advantage is going to be. Is your design station and printer room too far away or something? Are you just trying to get away from short USB cables? More reliable?