Originally Posted by w-v-m
Parallax would be horrendous
On the contrary it would be non-existent. Think about it: parallax is caused by different alignments of the eye (viewpoint) relative to the reticle and the target, when the reticle and target are focussed in different focal planes. Here the viewpoint (phone camera lens) is fixed rigidly in place, and the "reticle" is actually just pixels painted onto the image by software! Zero parallax error at any range. Clear focus from zero to infinity.
The phone would obviously have to be off centre so the camera lens is over the bore or it would have horrendous crossover.