As others have said, Qlab can output to it, though the OS will not recognize it as a display. Others have also commented that the lack of ability of the GPU to output to the device may be problematic. I can tell you from first had experience that this can be very problematic. I have a large show that was running on the BMD UltraStudio thunderbolt to SDI output that we ultimately had to replace for stability reasons. From a video perspective, we're not asking much of the system - it's playing back simple prores 422 proxy 720p files that are used internally by the musicians and stage managers (the audience doesn't see it). We had 30 or so relatively trouble free performances (albeit with some odd glitches), but ultimately we just couldn't get the system to be 100% stable. I resisted the notion of replacing the BMD device until every other option had been exhausted (causing a fair amount of frustration for the Qlab support folks along the way). After much troubleshooting, I finally relented and switched our backup system over to use the mac pro's builtin HDMI output and converted externally to SDI using a BMD MiniConverter HDMI to SDI 4K (not that I needed the 4K model). We have confidence monitors in the Qlab rack showing us the output from both the main and backup systems, and the difference between the two was stark after the change. On average the HDMI output was roughly 3 frames faster out of the system than the BMD UltraStudio. We do a lot of vamping and more or less always had a quick video glitch on the vamp exit - that was also cleaned up. The rest of our odd video issues were likewise solved (including a OS related memory bug that would crash us out of qlab). After a few clean shows from the backup system, we switched our main over to the same video output method and have been trouble free on the video side ever since.
If you're interested, here is a link to a video of the 2 confidence monitors. The monitor on the left is hooked up to the BMD UltraStudio, the one on the right Builtin HDMI to BMD MiniConverter HDMI to SDI 4K. Both Mac Pro's are otherwise identical following the same audio clock (the video is synced to the audio clock) and the Qlab files are identical. Apologies for the shaky camera work...
Best,
Andrew