My suggestion would to be to use a nylon coated beading wire that is at least a 49 strands at a thichness of 0.018" and has a rated tensile strength of at least 20lbs. Accu-Flex, Beadalon, and SoftFlex all have wire that meets those standards. Even if the beads are light weight I would usethe largest thickness beading wire the bead's hole can accommodate to counter the extra stress the motion of the foot flexing will cause on the piece. Also you could try a good quality rattail if the bead's holes are large, the flexibility of the fabric should work well with the foot's motion but may stretch out if if the customer gets it wet.