ok, so I understant that You wan't to use sleep functionality after all. You just wan't additionally to be able to cut off the power totally. So I see couple of problems with that:
- You will not be able to precisely keep track of the window state, unless you will send KEEPUP messages every couple of seconds which will make your node more power hungry.
- Be aware of battery self discharge. According to wikipedia the 9v alkaline battery will self discharge after 5 years. And it is very easy to make a window sensor that will last 5 years. So I do not see any benefit here.
If you realy want to add power cutoff to your design I would suggest to add some large capacitors that will hold the charge long enough after power cutoff to send an "off" message to the controller.