 Edge-on Schwarzschild ISCO redshift envelope

At $r=6r_g$, the local circular speed is $c/2$, so the receding direct-projection shift is $\sqrt{2}(1+\sin i/2)$. The upper envelope over inclinations is $3/\sqrt{2}$. It is not the shift for a face-on or arbitrary azimuthal emitter, and it excludes plunging-region emission and ray-tracing corrections.