[splat]

That's correct, lensing won't affect the observed redshift. It will, however, affect the observed luminosity of the quasar and make us perceive it to be more luminous than it really is. This would lead us to calculate a larger black hole mass. The lensing effect only goes as the square root of the magnification though, so you would need an extremely strongly lensed quasar to be able to explain the black hole mass. If that were the case, the lensing body would also be visible, which it is not.