To be fair, I haven't seen a good solution to this anywhere. Even on Apple's site, you'll sometimes be scolling down a page, and then the page will stop while an embedded frame scrolls sideways.

The pinch to zoom would work fine, but Safari intercepts this. Modifier key+scrolling to zoom would be fine as well, but not so discoverable. Apple should add some of the iOS multitouch events to Safari, but it's not a good fit since there's really only one visible pointer in OS X. Google Maps would really benefit from this.