Mick
I think the point that the two previous posters were trying to make is that having an automatic seatbelt release for ditching is a bad idea because the absolute last thing you want is for that belt to release before you're ready for it.
You need to be secure in your seat to identify your exit and then you need to be able to jettison and push the exit clear - it's incredibly difficult to do that if you're not still strapped into your seat - believe me - I've tried it in the dunker - All that happens is you push the window and it stays still while you float away. Hence the question of if you've ever done HUET.
As for a "timed release" as you suggest - I'm sorry but that won't work since this is not a fixed duration event. You need to be secure for as long as it takes you to stabilise and jettison. Then deliberately release your harness and go out your exit.
Automatically releasing all the pax, say, fifteen seconds after inversion will just turn the inside of the cabin into a wresting arena and everyone will drown. (and it will make the bodies more difficult to find!).
OH