It's a Nougat feature that has to be enabled. It's coming to Shield TV soon enough. Given that the Tablo Droid software DVR with USB tuner support is coming in Q2 for the Shield TV, I'd assume that's when the Shield TV will support USB tuners.
It is available now on the Nexus Player. AirTV will add it in their own special way. I have no idea what Xiaomi will do.
I have tried a hauppage USB tuner with the Nexus player via a powered USB hub connected with an otg cable. Can confirm that it doesn't work sadly.
Anyone got this working in the UK?
Because TV Tuner support in Live Channels is a collaboration between Google and SiliconDust, not Hauppauge. Different drivers.
Edit: not 100% certain on this, but that's my hunch.