Actually, I can pinch-to-zoom using my thumb and pointer-finger on the screen with my remaining three fingers holding the phone. It's awkward, but works in a pinch (haha).

I much prefer a gesture that makes intuitive sense. Pinch-to-zoom is very easy to learn and just seemed right the first time I saw it. I haven't tried the double-tap+scroll gesture you mention, but it sounds like something that would be a tad trickier and less obvious.