External automorphisms of ultraproducts of finite models

Let L be a finite first-order language and 〈M _n {pipe} n < ω〉 be a sequence of finite L-models containing models of arbitrarily large finite cardinality. If the intersection of less than continuum-many dense open subsets of Cantor Space ^ω2 is non-empty, then there is a non-principal ultrafilter U over ω such that the corresponding ultraproduct Π _U M _n has an automorphism that is not induced by an element of Π _n<ω Aut(M _n).