I've been working on the hardware side of things.
Signal runs on an Arduino Uno and communicates wirelessly using a Bluetooth v2.1 module I picked up from ebay. It needs to run on battery power for a long time - about the same or even longer than a regular television remote would last (roughly 6 months).
The plan is to put Signal (the Arduino) to sleep for most of the day, when people aren't watching TV (or even in the house). That condition is determined by the server, and it sends sleep/wake-up commands to the arduino. Combined with a high amp-hour, rechargeable lipo battery pack, this ~should make the arduino last for quite a bit.
However, since the actual client-side software isn't very complex, I've been thinking of programming straight onto a bluetooth chip - that could possibly cut down a lot of power consumption. I'm not entirely sure how to go about doing that, but it is something to think about.