  We discuss the effects of resonant spin-flavor precession (RSFP) of Majorana
neutrinos on heavy element nucleosynthesis in neutrino-heated supernova ejecta
and the dynamics of supernovae. In assessing the effects of RSFP, we explicitly
include matter-enhanced (MSW) resonant neutrino flavor conversion effects where
appropriate. We point out that for plausible ranges of neutrino magnetic
moments and proto-neutron star magnetic fields, spin-flavor conversion of
$\nu_\tau$ (or $\nu_\mu$) with a cosmologically significant mass (1--100 eV)
into a light $\bar \nu_e$ could lead to an enhanced neutron excess in
neutrino-heated supernova ejecta. This could be beneficial for models of
$r$-process nucleosynthesis associated with late-time neutrino-heated ejecta
from supernovae. Similar spin-flavor conversion of neutrinos at earlier epochs
could lead to an increased shock reheating rate and, concomitantly, a larger
supernova explosion energy. We show, however, that such increased neutrino
heating likely will be accompanied by an enhanced neutron excess which could
exacerbate the problem of the overproduction of the neutron number $N = 50$
nuclei in the supernova ejecta from this stage. In all of these scenarios, the
average $\bar\nu_e$ energy will be increased over those predicted by supernova
models with no neutrino mixings. This may allow the SN1987a data to constrain
RSFP-based schemes.
