Also, being able to do so, straight from the TeamViewer Computers & Contacts menu or even from the online dashboard, without the need to actually initiate a remote session would be very nice. I see many applications where this would be extremely useful and also would save so much time ;)
It's not the prettiest solution, but this works:
1. Make sure Teamviewer Host is on INTERNAL/PHONE storage, not the SD card. (If it's on the SD card, Teamviewer can't auto-restart).
2. Install the app "Button Savior." (the non-root version works). Start up Button Savior and turn on the Accessibility Services. Now, there will be an overlay on the right side of the screen that will pop open a series of icons for hardware keys, including the POWER button. You can press/hold that icon as if it were the hardware power button, and reboot the phone.