What you can do is adding a dummy device or what is called on this forum a pseudo device [Link: remotecentral.com]
This is not a device which you actually have but just one (preferably similar device as already in the activity, but different brand) you add to the activity. You then program the power on button for this device with the warning/wake-up signal. If the activity is powered on the dummy device will generate the wake-up ir-code for your activity.
I hope this inspires you and good luck