Hey Monkey kiter,
Heel bungees are what you are describing, they work reasonably well and will help to hold your feet into a footstrap that doesn't fit particularly well.
Many modern boards have infinately adjustable footstraps and contoured deck pads which remove the need for this kind of setup. There is no major accident risk as long as they are not too tight as to rub your heel into sea ulcers. Your foot will normally come out before your ankle will break unlike bindings when one foot only comes out.
You can buy proper heel bungees or just use an inner tube from a bike as you have mentioned. Best idea is to experiment with an inner tube version and if you really like it, maybe buy the real thing if it is more comfy.
Good winds,
