I have a similar problem with different behavior....
I have a custom registration page that uses post back, and inherently uses webresource.axd. If I am logged into DNN, everything works fine. If I am not logged it I cannot access webresource. If I view source and copy the url for webresource.axd to my browser I am redirected to my login page. After I log in the file is accessible.
I already have the IIS settings done and DNN does not compress this file. I am running DNN 4.4.1, and cannot upgrade the version prior to my next release. Any thoughts on how to change DNN permissions on webresource.axd?