I like the nylon string saw idea! neat trick! Nylon string or rope gets little frition nodules the more its used hence it will gain more friction. (I'm sure you could find a better description on a website discussing moutaineering rope).
You might also consider using piano wire and letting it rust in salty water to make it more 'saw' like. Piano wire is cheap and comes in more sizes than you'd need even in a piano. (WW Grainger good source)

One of the orginal questions was Are there different types of hot melt glue? Indeed there are. Craft hotglue sticks I think are the 'weakest'. Be worth checking out and doing a few tests. There is also now a new hot glue system that is far more permanent than what you'd want for temporary attachment to forms. Make sure you don't spend a lot of money on the nifty new system that won't let go when you need.