kahuna, > But the server is returning 403.
... And that response is incorrect and you should find out why. Just because Google has a work-around, doesn't mean that others do. And maybe their work-around will be accidentally removed some day... Unlikely, but do you want to bet your business?
I strongly suggest you find out why your server is incorrectly returning a 403 server response and fix it.
If your site does not meet HTTP specifications, it is a disaster waiting to happen. 403 has a meaning and 404 has a meaning, and they are definitely not interchangeable! Also, put up at least a blank robots.txt file and avoid all those 404's in your logs!
Jim