I'm far from an expert in ComputerCraft, but I think the real stumbling block is the "if I fall below y level 0" part. I don't think there is anything that simply detects player location, even with OpenCCSensors. The best I can think of is making a GPS system, which needs to cover the area you are at, then having the pocket computer constantly polling the GPS location. ( I'm not even sure how the pocket computer works with that. I haven't gotten around to CC on current versions. )
The redstone signal to a command block would actually be very easy.
A workaround might be to have some sort of input from the player. For example, if you fall, pull out your pocket computer and turn it on. Then, simply have the teleport script named startup. As soon as you start the pocket computer, it sends the signal, and you get teleported. However, this has the obvious downside of requiring you to be fast enough to react to the situation.
If you have a keyboard with macros, you could build one that will teleport you to safety.