the only way i can think of this working is if you convert the metaclay to polys. that way qstretch should work fine.
i dont think it would be possible to get the desired effect another way becuase this usually involves deforming the object via it's vertices/control points. these do not exist in meta clay elements as they are made from iso-surfaces (a field equation) and are only rendered as polygons so that it is visible.
sorry i cant be of more help
leach.