Obviously, Apple decided in its infinite wisdom that digital audio in via Toslink has to die, like the Floppy Disk and SCSI earlier in the course of the history of Apple. So future audio interfaces and DACs will have to offer Firewire or Thunderbolt connectivity to be able to interact with the new MacPro. As an Apple user, I am used to the "Apple giveth, Apple taketh away" behaviour of the company, so no problems there :-)
Thankfully, thanks to my own infinite wisdom, I opted for the Firewire card in the Rosetta 200 when I bought it (which I almost had forgotten about), so my problem is solved: I attach the DAC via Firewire now, which works fine.
On the other hand, there seem to be no standalone Toslink to USB audio in solutions on the market. Sure, there are music interfaces (as mentioned before) with much more extended functionality that could serve this purpose, but as a highend audio user, I strive for simple solutions with quality components.