I've toyed around with some ideas for swipe-like mechanics, but none of them worked well, I'm afraid. They all felt somewhat unnatural in a way, and were either too jumpy or not responsive enough.
However, what would be possible and perhaps more suited to tablet / touch mechanics than the pseudo-mousemove, would be some kind of "tap" rather than "swipe" mechanic. I.e. one could have two <shape> elements in the background (they need not be visible, but can be), one for "push" covering the upper half of the screen, the other one for "pull" covering the lower half of the screen. Tapping anywhere inside the upper screen half, then, would "zoom out" the target stimulus by a fixed size. Tapping anywhere inside the lower screen half would "zoom in" the stimulus by a fixed size, and people would have to tap repeatedly until the target is either fully zoomed out or fully zoomed in. Might this be a viable alternative for your purposes?