I'm a little confused here. What would be the advantage of Firehawk connecting to Workbench? Perhaps it depends which model Variax you have? I'm no expert, but here's my take on this...
Unless I'm missing something, Workbench is basically a custom model editor for the Variax, while Firehawk is an effects unit which also has the capability to interact with the Variax. Surely, if you are making tweaks to your Variax custom models on Workbench and you want to hear what they will sound like through the Firehawk, you should just plug the Variax into Firehawk via a 1/4" cable and listen on headphones while you make your edits in Workbench. My Variax Standard connects to Workbench via the Cat 5 cable, so it is easy to run both connections at once. Obviously, you won't be able to use the Firehawk to recall guitar models from the Variax at this point, but you wouldn't be wanting to do that until your new custom models are saved to the guitar anyway.
That's my understanding, but like I say...I'm no expert!