  We construct a martingale observable related to the spin interface for a
class of non-integrable variants of the two-dimensional Ising model and express
it in terms of Grassmann integrals. Under a conjecture about the scaling limit
of this object, which is similar to some results recently obtained using
constructive renormalization group methods, this would imply that the
distribution of the interface at criticality has the same scaling limit as in
the integrable model: Schramm-Loewner evolution SLE(3).
