Yes, a lot of cable remotes don't let you change the power button, so you'll just have to pick another button. Any other key besides a device selection key is fine. I think Shift-Power should work. To do that just precede the power button with a short press of setup when programming it. Use it the same way, by preceding it with a short press of setup. Using the setup key this way is called a shifted function. You can pretty much double the number of functions on the keypad with this technique.
, setup followed by a 5 digit code should work on your remote. You have to do it fairly quickly, all 6 presses within 2 or 3 seconds. You can hold the final digit to keep sending the signal. I've had a few atlases in the past, including the 1056, and never had an issue like that.
If you don't mind investing in a JP1.3 cable, you can reassign the power button as well as do lots of other things like 5 functions per button, including device buttons, such as short press, long press, double press, shifted press and double shifted press, and do nested macros, fast macros, device specific macros (as opposed to global), key group mapping, device multiplexing, variable macro pauses, device state tracking, load new devices and protocols, import pronto hex, etc., all using a graphical programming interface similar to harmony (but much better).