When you've created your overlay in a perspective window, the only way to zoom in and out from that view is to use a Paint window.
When you are in a paint window, you can select the overlay plane from the layer editor to bring it up. This allows you to have a normal zoom behavior in 'perspective' views. Otherwise, there is no way to adequately get this behavior in a perspective window because of the conflict with zooming in 3D space (and losing the overlay, changing the camera perspective, etc...).
Hope that helps.
c.