Actually, this could be done, i don't got a link, but a while ago i saw this proof of concept that worked based on recognizing the jump the cursor does when you touch a second finger on the screen, and then extrapolating the position of the fingers based on the cursor position (it goes to aproximatly the point between the two fingers). It wasn't exactly like true pinch zoom though, you can't scroll at the same time and you should only move the second finger and keep the first one steady; but rotation should still be possible, as long as you just move the second finger.