I think QUATERMASS is on the right track. Design a new mouse which incorporates a trackball or wheel, which you may scroll and/or zoom (similar to the MS-mouse Scroll Wheel). Then your hand never leaves the mouse. It could be a small trackball which pushes down to zoom in, and a small button nearby, which will zoom out.
I think the whole zooming idea is good, but it needs some designing. Please excuse me if the mouse idea has already been mentioned/produced.