Could you use the web pinger app?
Assuming you don't have a UPS on the router or that you have one with a lower time service than the one on your HE, you can test if the website is available time by time (say every 5 mins), if it goes down you can take that as a reasonably strong proxy that the power is down (of course it may be the site is down, but if you use google.com or suchlike I guess it will be quite robust). Then if no response, wait a while (a bit less than the duration of your hub UPS service time), then check again, and if still no response, power the hub down with an https post.
Update: I like this idea enough to be trying it. I notice also that web pinger has a timer check (normally set to 2 mins) but I'm thinking this could potentially be set to 3 hours worth of minutes and Bob's your Uncle, as we say from the UK. Will report back how I get on...
