According to this post the cookiejar file can consist of raw HTTP headers.

1) Is this an officially supported feature that will continue to exist in future versions? (The only reference to it that I could find was this one forum post.)

2) Does this need to be limited to Set-Cookie headers or will Prince simply ignore non-cookie headers (e.g. Content-Length)?

Technically this is a feature of libcurl that we are using for HTTP support, so it may change in future releases or depending on which version of libcurl is installed, although I don't have any expectation of it changing at this time. I'm not sure what meaning any non-cookie related HTTP headers would have in the cookie file.