It all really depends on how sensitive her feet are dude.
If her feet are sensitive enough to be feather ticklish, then buying and then using a Go-Duster while she's tied up. Other possibilities include a car wax buffer with a fluffy soft wool pad.
If she's not feather ticklish, try using a wartenberg wheel or just your fingers.
If she's not sensitive enough to be ticklish at all on her feet then you might be out of look getting your fetish satisfied.