All,
I emailed SmartThings about this and received the below reply yesterday. While I didn’t try it, it looks like this will allow us to reboot from anywhere. It would be great if this could be done automatically. Someone smarter than me can probably write some PC-based code to check for the hub presence initiate the reboot via Python, etc if offline for x minutes/hours.
I edited their instructions since it missed a step if you only have one hub:
Hi Alan,
Thanks for reaching out to us! Yes you can do remote reboot using the [SmartThings web manager].(https://graph.api.smartthings.com/)
Follow these steps to reboot:
- Log into https://graph.api.smartthings.com/
- Click “My Hubs”
- Select your Hub. (Edit: If you only have one hub, click on “View Utilities”. It’s at the very bottom and isn’t blue/underlined, but is a hyperlink)
- Scroll down to the “Hub commands” then click " reboot Hub
- Then click “My Hubs” on the navigation bar
- Select your Hub
- Scroll down click events
- You will see the Hub disconnect, then a few minutes later it will reconnect.
Alternately get in touch with support and we can do it for you. 
Cheers,
Sam