I'm a little confused as to what you want. The product pages in your store are now displaying custom information.
Here's an example of a non-store 404 page page on your site: http://rabbitlvr.com/qwer/zvxcz/?asdf=1234
Nothing in the URL changes if it's a 404. A 404 is served but the extra information is displayed. Isn't this what you want?