  Searches for heavy neutrinos often rely on the possibility that the heavy
neutrinos will decay to detectable particles. Interpreting the results of such
searches requires a particular model for the heavy-neutrino decay. We present a
method for placing limits on the probability that a tau can couple to a heavy
neutrino, |U_tau4|^2, using only the kinematics of semi-leptonic tau decays,
instead of a specific model. Our study suggests that B factories with large
datasets, such a Belle and BaBar, may be able to place stringent limits on
|U_tau4|^2 as low as O(10^-7 - 10^-3) when 100 MeV < m_4 < 1.2 GeV, utilizing
minimal assumptions regarding the decay modes of heavy neutrinos.
