Not sure you can do that (yet?). Believe me i tried. You can scroll in and out (mouse wheel) but not up and down on a vertical plane.
Yes, I misunderstood OP's question. However you can get close to eye level and high up by scrolling. You can get strange effects if you move the camera using the keyboard, but it can be very disconcerting if you move over a slope as you see the top of a rock as a line with more rocks behind.
An x, y, z system would be greatly appreciated.