I got my Tripp-Lite Keyspan USB-to-serial adapter (the one JVC recommends) and a Tripp-Lite null modem cable from Amazon today. I used them with my 2012 15" retina MacBook Pro running Windows 7 x64 through Boot Camp and the upgrade finally went off without a hitch.
Things of note:
1. The Startech adapter that's been recommended a few times did NOT work with my configuration. It might work with yours but it's easier to avoid headaches and buy the one JVC recommends.
2. Make sure you know what COM port your adapter is using; you'll need to know this for the first part of the update. You can see this by expanding the entry under Control Panel > Device Manager > Ports (COM & LPT). Mine was COM4 FWIW.
3. Make sure the baud rate that the first firmware update program uses (57600 specified by JVC) and the baud rate specified by your adapter's settings match. Again, go to Control Panel > Device Manager > Ports (COM & LPT), expand the tree, right-click on your device, click "Properties," click the "Port Settings" tab, then select 57600 from the "Bits per second" drop down (thanks Seegs!).
4. Don't forget to install the drivers for your adapter!
Other than that it was easy. In fact, if you have a good adapter, you shouldn't have any trouble. IMHO don't waste your time with the Startech adapter.