Actually, it would be much smarter (and imho easier) to implement the spinning gesture on a device driver level. it should simply send buttons 4 and 5 (mouse wheel up and down), just like synaptics touch pads would on edge-circular scrolling. That way it would be universally implemented and really any app would already have scrolling implemented using this spin gestures. Devs would then only need to care about remapping input for buttons 4 and 5 to zooming instead of scrolling.

But I'm unsure if the community has any influence on the touchpanel device driver.