  We explore the second order bilinear magnetoelectric resistance (BMER) effect
in the d-electron-based two-dimensional electron gas (2DEG) at the SrTiO3 (111)
surface. We find an evidence of a spin-split band structure with the archetypal
spin-momentum locking of the Rashba effect for the in-plane component. Under an
out-of-plane magnetic field, we find a BMER signal that breaks the six-fold
symmetry of the electronic dispersion, which is a fingerprint for the presence
of a momentum dependent out-of-plane spin component. Relativistic electronic
structure calculations reproduce this spin-texture and indicate that the
out-of-plane component is a ubiquitous property of oxide 2DEGs arising from
strong crystal field effects. We further show that the BMER response of the
SrTiO3 (111) 2DEG is tunable and unexpectedly large.
