• Defining your own Accept* HTTP headers. Please just give me a set of text fields so I can write my own headers! Especially Accept-Language, that's the most important—my system language is set to British English, yet Safari sends out requests with "en-us". :-/

If Safari sends out an accept header that is inconsistent with your system language setting, that seems like a plain old bug, and not something requiring a new feature to work around. It would be helpful if you filed the bug @ bugzilla.opendarwin.org, along with steps to reproduce it, and a comparison to other browsers' behavior.