For all the trouble, seems like working the friction aspect is much more sensible solution, if you can find a way to put some type of lubricant (that dirt and everything else wont stick to).

Springs wear over time, always have always will. Laws of physics, spring tension doesnt last. You can tighten it and tighten it, but its a metal spring, eventually it will fatigue again. And the seatbelt isnt someplace that I would want to find out about it.

Just another 2 cents...