  We investigate boundary multifractality of critical wave functions at the
Anderson metal-insulator transition in two-dimensional disordered
non-interacting electron systems with spin-orbit scattering. We show
numerically that multifractal exponents at a corner with an opening angle
\theta=3\pi/2 are directly related to those near a straight boundary in the way
dictated by conformal symmetry. This result extends our previous numerical
results on corner multifractality obtained for \theta < \pi to \theta > \pi,
and gives further supporting evidence for conformal invariance at criticality.
We also propose a refinement of the validity of the symmetry relation of A. D.
Mirlin et al., Phys. Rev. Lett. \textbf{97} (2006) 046803, for corners.
