I just tried Nightflier's suggestion and it worked for me.
I was just browsing (not in root) and saw your post. I then opened a new browser page in Seamonkey and typed:http://localhost:631/admin
in the url and Cups came up.
I then clicked on "set printer options" and the proper screen appeared.
I likely would have been required to log in as root if I really wanted to change the printer options (I did not try that), but from your post, may I assume that you are doing the exact same thing (see above) and still getting the 403 error when you click on "set printer options?