I've spend a fair bit of time contemplating safety gear as well. I think that the rapid air concept is generally a good one that has not successfully been integrated into a kayaking specific format that is truly workable.
I think spare air is closer...
Spare Air - the smallest redundant SCUBA system available with enough air to get you to the surface in an out-of-air emergency.
Main issues are...
1) it has to be accessible, preferably hands free
2) you have to be able to breathe with it while you are working to extract yourself (big ding on the rapid air vest pocket deal)
3) it has to have enough air capacity to provide a meaningful amount of time
4) must be small enough to not be overly heavy, overly bulky, or present entrapment hazards.
I think that there is certainly another level of evolution that could be done. I have envisioned a hybrid between a whitewater rescue PFD and a scuba BCD that would be an integrated vest with holders for small air cylinders, HP regulatory, and LP regulator / mouthpiece. I like the concept of the BCD spare regulator that has a couple ft of reach.. ie enough length for primary user to breath out of, or secondary pinned victim to breath out of.
Really cool design would make it integrated into creeking helmet.
Biggest challenges I see are that 99% of kayakers really don't need this, and would likely not pay for it unless it was pretty cheap. Not a huge market, thus the limited R&D done to date.
If I had a wish list... it would be an expert kayak gear system... PFD with small air tanks and regulator on back, flex hose quick connect connecting to helmet with mouthpiece embedded, perhaps add in helmet to helmet radio connection.
Flip side to all this, is that systems fail and can produce chain reaction failures or other problems. There is a really good arguement to keep kayaking simple. Many severe swims, pins etc would not necessarily end differently with another 5 mins of air.