Spontaneous ferromagnetism occurs when exceeding the "Stoner limit", a condition requiring the potential (Coulomb) energy to exceed the kinetic energy cost of splitting the spin-up and spin-down bands. Some materials (such as Pd or Pt) are on the verge of ferromagnetism, with enhanced paramagnetism but fall below the Stoner limit and thus have no long range order. Compounds of the form RT2Zn20 are mostly non-magnetic Zn, but the rare earth ion R and transition-metal ion T can have a profound effect on the magnetism, allowing the Stoner limit to be studied. We propose a muSR study of RT2Zn20, thereby gaining insight into the development of ferromagnetism. This problem of a non-magnetic host with moment-carrying species in a dilute array is somewhat reminiscent of that of spin glasses, in which muSR has made important contributions, though here the moments are structurally ordered.