  Type IIA orientifold constructions with intersecting D6-branes and their IIB
duals in terms of magnetized D9/D7-branes constitute one of the most promising
avenues for the construction of semirealistic MSSM-like compactifications. One
generic problem with these constructions is, that there are many Yukawa
couplings, which vanish due to additional U(1) symmetries in the theory. In
this paper we consider a number of such settings and study, under what
conditions stringy instanton effects can give rise to non-perturbative
contributions to the Yukawa couplings, so that all perturbatively forbidden
terms are induced. We find specific settings, in which indeed Yukawa couplings
for all fermions are obtained. For some cases we provide specific local
examples of rigid O(1) Instantons within the T^6/ Z2 x Z2' toroidal orientifold
with torsion, giving rise to the required amplitudes. A potential problem in
these settings is, that the same instantons, providing for Yukawa coupling
contributions, may give rise to too large mu-terms for the Higgs multiplets. We
show how this problem may be overcome in explicit models with a doubled Higgs
system.
