I've been interested in this too. I've thought about it. There are some retractable cable-type units used in construction that might be able to be adapted to a somewhat functional grappling hook line. You'd need a lathe to turn a decent collapsable grappling hook I think. I usually work in wood and plastic, not metal. I need to ask my local blacksmith how he'd make it. Still, I'd be interested in getting one too.
Don G.
TK-5372