A site that supports rss feeds seems to be negotiating based on user agent and instead of serving me my RSS they instead send me to their "mobile" login page.

This is quite annoying since my browser on the E90 isn't a dumb browser and I want it to act just like my Safari browser WRT to these RSS feeds.

The built-in browser doesn't let you do it. I don't think Opera does either (haven't tried their S60 3rd Edition version, but earlier versions didn't, if my memory serves me right). Other than that, there's only Opera Mini.

Tried also contacting the site admins in question and asking them to change how their site behaves?

Did you try to add that rss website to nokia internal rss reader that is found in the index?

For me works like a charm.

just add the web page and the rss reader downloads latest news. and this include also thi site :)