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Just as jamch said, the HDMI passthrough works fine and you control TiVo just as you would normally. There's a catch though, the Xbox has to be on for this to work. Switch of the Xbox and you lose the TiVo picture too.
At the moment, since the two aren't really integrated, I probably won't use the passthrough feature and save that precious electricity. But once voice control etc start to work for TiVo (who knows when?), then I'll try again.
By the way the One controls my Toshiba TV just fine, I can say volume up or down and it does just way I say. Though pressing a button on a remote I think is still easier!