  A light non-standard Higgs boson decaying mainly into $\tau^+\tau^-$ has not
been yet ruled out by LEP searches in several scenarios. We verify that, in the
context of the Next-to-Minimal-Supersymmetric Standard Model, a low-mass CP-odd
(mostly but not completely singlet-like) Higgs boson can couple strongly enough
to down-type fermions to be detected in radiative $\Upsilon$ decays into
tauonic pairs at a high-luminosity B factory. Possible spectroscopic effects of
a mixing with $\eta_b$ resonances are also analyzed
