Site-specific User-Agent Safari
Here’s a two-year-old topic on Quora asking the same question. No answer yet.

This leads me to believe there is a way to do what you want. Always changing the UA string using the menu is annoying, but at least it works.

The Opera web browser has a feature that does what you want. But of course, if you have to install a separate browser anyway, you may just as well install Firefox 3.6 and use that to access the site you’re talking about.