If you are using the default image to zoom to in IH then it really is not that difficult. (http://goo.gl/GHV7L) IH will create the medium image as the page loads. The default image ( the zoom-to-image) exists because it is the one that you uploaded. This requires the user to be disciplined on the proportions and size of the images that they upload. (Or to put in place a system that scales the image on upload which is a better solution because it stops huge images being stored on the server and never used)

If however, you allow Image handler to create a 'large image' and you want to use that as the zoom-to-image ( which kind of makes sense because it means that the zoom ratio is constant) then you have an issue because the image may, or may not, exist on page load. And the javascript cannot create it. So, as I think I may have mentioned before the solution here is to pre-load the image somewhere else on the page and hide it but use it's source in the zoom tag.