That should work, although I would recommend starting with a stiffer spring from a hardware store along with a little piece of rubber or sponge to add the progressive feeling.
My first brake mod was basically made up of those items. The trick is to find the right softness along with the proper length of rubber/sponge to create the right feel. The length is important so it gets stiffer when you want it and not too soon.
If you get it right it can feel close to a progressive brake spring, although it will wear out over time (not a big deal).
