I remember reading something Badspot posted about this. He tried it originally, but if you have so many people walking around eachother, it would sound just like a mess. I think he decided against footsteps.
It would be cool to have the people using the invisibility watches have footsteps, so you could catch them by hearing them. I would love if anyone made this mod.
Another thing, you could make it toggleable in RTB Prefs.
OMG i had the same idea! great minds think alike
Another thing in the RTB Prefs for it, is you could change how loud your footsteps are, and how loud other peoples footsteps are.