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.
If lensing (through one or more lenses) resulted in a significantly longer path for the light, would it not make sense for an object to seem older than it is (i.e. have greater redshift)?
Lensing will make light travel through only a very slightly longer path. At most the difference will only be a few light-weeks. (This is seen in objects which have multiple images. Occasionally you will see one of the images flare up for some reason, and a few days later the other image will flare up.) It won't be anywhere near long enough for cosmological effects like redshift to be important. The main effect of gravitational lensing is just to increase the apparent luminosity of the lensed object.
Thanks for the clarification. I was imagining a situation that would require tens of degrees of deflection, with two or three lenses zig-zagging light across a cosmologically significant distance.