bicycle clips will not come out unless you twist your ankle outwards. They're also designed for you to pull up on the pedals if you need some more power climbing hills. If you absolutely had to stay in your tray bicycle clips would be the solution. you coming out of your clips would be a non-issue.
I've considered magnets in shoes too. Probably doable. The idea being the magnets would just hold your heal down. Your toes would be under some sort of foothold, and the back of your heal would be against something. Magnet would be easy to slid off, but not easy to pull straight up out of.
The best holds I've seen as far as not ever being able to fall out are the ski clinic holds w/heal block. I wish they included them on their pro carbon hulls to customers, but they don't. It's the thing I wish the hulls came with. I never asked them why, but they just don't. The only way to get out of the clinic holds are to twist your ankle inwards. Exactly like you had on a pair of bicycle clips, except the ankle must come inwards to get out. Most other holds are some variation of padding and kicking your foot into them as hard as you can.
