AF point as capture trigger?

I'm wondering if I can use the central AF point of my 450D (or other Canon DSLR) to automatically take the picture as soon as it detects focus (as when the AF point lights up in the viewfinder)?

I know this is a bit strange question, because normally one wants to pull the trigger at the right moment, and not as soon as a subject is in focus. But it could help for one of my subjects which is photography of dragonflies in flight. AF is way too slow for this (at least with my equipment), dragonflies can move at 5-15 meters per second and at the short distances needed for closeup shots, there is no way the camera and lens can keep up. So I'm now using manual focus, setting a suitable focus distance and hoping the dragonfly will cross at the right spot, and taking the shot when I see the subject in focus using the optical viewfinder.

I read about someone using laser triggers for this, but wouldn't it be possible to automatically fire the shutter as soon as the AF point detects a focus lock? The 0.2-0.3 sec delay of having to press the shutter myself will ruin most images because of the fast movement, but taking the photographer out of the loop would speed things up. Of course this way you get many boring images, wrong composition etc. but maybe also the right lucky shots

Maybe I'm overlooking a function in my camera that I could use for this, or maybe I need another DSLR model (or special firmware like the Wasia / Magic Lantern hacks?). Externally controlling the camera with a tablet or something is not a good option for me, because the camera needs to move freely and usually is not mounted on a tripod. Some typical scenes below: