I'd like to throw my hat in the ring, that this would be a great feature to bring to the Almond+.
I have used the Piper home security system for over a year now, and just purchased a Canary as I'm thinking of switching to that platform. Just recently, my ISP had hours-long outages in the middle of the day, 2 days in a row. This was while I was at work, and both of the security systems were essentially useless during the outages, as you'd expect. These are critical services for the home these days, so if you want to take advantage of these new home security products, you need dual WAN.
Just a simple fallback/backup using a WLAN USB dongle would be perfect. Here in the US, mobile broadband prices are still relatively very expensive. With a simple fallback, you'd only be using very little bandwidth until the primary connection comes back up, and only very rarely. Something like T-Mobile's data plans that accumulate what you don't use for the past year would really work well with this situation. If there is an outage with your primary and you're home, and maybe were just about to stream a movie, you could still do that on your backup WLAN connection as you've accumulated many GBs on your plan.
