Doing a quick scan of what's out there it appears that it's pretty trivial to find RS-485-to-USB UART dongles, but the problem is that the "RS" part of both of RS-485 and RS-422 define the *electrical* characteristics of the interface, and those dongles inevitably seem to have very "simple" UARTs attached to the line drivers. Add the ability to do SDLC processing like the Zilog chip and you end up with shockingly expensive things like this..
I still like the idea of interfacing the Z85c30 to a USB capable microcontroller instead of doing it as a shield directly bit-banged by something like a Beaglebone. In theory at least you could offload some of the packet processing overhead to the MCU so the driver for the host OS (be it Linux, BSD, whatever) could be abstracted to a really simple character device. Stuff a complete packet in, pull a complete packet out when one arrives. No spinning your wheels bitbanging a device designed to sit on a "real" CPU bus and the dongle could be attached to the Linux-driven NAS box that a user might already have. But of course that comes with all the requisite handwaving about making it work, etc.
I still like the idea of interfacing the Z85c30 to a USB capable microcontroller instead of doing it as a shield directly bit-banged by something like a Beaglebone. In theory at least you could offload some of the packet processing overhead to the MCU so the driver for the host OS (be it Linux, BSD, whatever) could be abstracted to a really simple character device. Stuff a complete packet in, pull a complete packet out when one arrives. No spinning your wheels bitbanging a device designed to sit on a "real" CPU bus and the dongle could be attached to the Linux-driven NAS box that a user might already have. But of course that comes with all the requisite handwaving about making it work, etc.