Hate to throw water on your campfire, but setting up a good, USB connection (only) for real-time audio data transfer can be problematic enough requiring optimum buffer settings and a good, bug-free driver. Adding a RS422 serial converter to the mix is (IMHO) just asking for trouble. Even using a USB hub in the data path can be an issue depending on what other devices are on the hub.
Those RS-232/422/485 to USB converters
were intended for use with printers and scanners which have large internal buffers and don't require real-time data transfer. What kind of a device do you have that outputs AES digital audio via RS-422? Is the RS-422 device something that is core-audio compatible, or do you have a driver that will work with it?
Having said that, the only way you'll really
be able to find out if it might work is to try. Maybe you can get a serial converter with a return policy if it doesn't work.