Yea, I know people at Microsoft that have there 360 plugged into their Xbox One. So if they can do it and know the hardware, it should be fine.

The input lag isn't just from the Xbox One. Your tv has input lag, the 360 has input lag for the controller, then more depending on the game, and then the Xbox One will further add input lag. So you might have 120-200ms without the Xbox One acting as a pass through.

I have my PC plugged into my X1. The lag is VERY VERY high. about 200 msec. Playing X1 itself has ZERO input lag. Since this "connection" feature is designed for watching TV, i believe the X1 does some preprocessing, before sending the data back to the TV. The input lag is super annoying when navigating with the mouse on a PC connected to the X1. What the X1 needs is a "PC" feature for the HDMI In as Option to remove all preprocessing when receiving data through HDMI In and sending them back to the TV immediately.

And dont confue X1 Input lag with a second device connected to X1 input lag, because the X1 by itself dont have input lag.

Took the initiative to do a video about it. I played a match of MW3 to test playability.
check it out:

https://www.youtube.com/watch?v=DHwbxACX3HQ