They have a similar setup for motorcycles, it's essentially a motorcycle airbag. Rider wears a vest with a ripcord attached to bike. Rider runs into something, flies off bike, rip cord gets ripped, vest inflates, and ideally no more injuries to the torso. I don't think they've reached the market yet, but I imagine they'll be unpopular, for the same reason your proposed device would be. It'd probably end up being a one shot deal, if the cord gets pulled the whole unit has to be reset (or factory serviced), so what happens when you space out and get off the bike without thinking?

Unfortunately current helmet designs are not the safest, a full face helmet with a HANS device would be the best, but nobody is going to use that on a bike. I would think they're reached their current state cause people don't like several pounds of helmet on their head, and more importantly, everyone wants to look cool right?

I'd follow MichaelW's advice, see what the most common injuries are, and make something to prevent it.