IIRC, there's no direct way to do what you're thinking of. Take this with a grain of salt, as I've never actually actively used these gestures outside of quick low-level input testing, but my understanding is that pinch/spread is only bound to *font size* scaling, and that on *reflowable* documents; so *not* what you're using.
Assuming it's all plugged into GestureManager properly, you miiight be able to get away with something like binding spread to a larger zoom factor and pinch to a lower zoom factor (which whould require being in manual zoom mode to begin with), but I'm not sure it's actually doable, and I wouldn't expect it to work all that well, *especially* on a low power device like the Forma.
Take all this with a grain of salt, my own experience with fixed-layout on that device involved sticking to perfectly screen-sized content, so I don't have to deal with any of this nonsense
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As mentioned earlier, though, if you're actually in the image viewer, e.g., by long-pressing on an image [which will work much better on actually reflowable content, e.g., ePub], then pan/spread/pinch will work *exactly* like that, though, and we've done extensive tweaking to make this behave mostly sanely on slow devices.