Since you're using this with one hand, tapping anywhere other than the area where your thumb naturally rests would be uncomfortable.
Not any more uncomfortable than single tapping any other part of the screen one-handed. Of course this depends on screen size...
what if you wanted to zoom in to a position near the top of the screen?
Point well taken.
Assuming that the content of interest is a good centimeter or so in from the edge, you could map that point-to-edge distance to cover the remaining zoom scale. If it's inside that centimeter, zooming into the middle is useless anyway.