I had the same problem and found a work around to enable art mode via the apple tv siri remote. You can't program the power button to a specific IR code, but you can program the volume up, volume down, and mute button. If you are willing to give up the mute functionality of the remote go to your apple tv settings, remotes and devices and program a custom IR profile for volume controls. Set up the volume up and volume down buttons with whatever you want (probably just the value from your Samsung oneremote, or your receiver remote), when it asks for the mute button use the power button on your Samsung oneremote. Now when you press the mute button it will turn your tv into art mode as if you used theSamsung remote which should send a signal to turn off your apple tv via HDIM-CEC from your tv. It's not the best solution, but if you're dead set on only using one remote I think it's the only solution outside of Samsung releasing a firmware update for CEC control to art mode.
... View more