Accessing req and resp header values

[Alexander]

I am trying to implement a bit of logic to change the Content-Type of the
response, but part of the logic requires knowing the User-agent, which is in the
request.

Example:

sub vcl_deliver {
    if (req.http.User-Agent ~ "MSIE 6" 
        && resp.http.Content-Type ~ "application/xhtml+xml") {
        set resp.http.Content-Type = "text/html";
    }
    deliver;
}

Result:
Variable 'req.http.User-Agent' not accessible in method 'vcl_deliver'

I haven't found any documentation stating what variables are available in which
functions, but so far I haven't been able to figure out any way of doing this
short of messing with the hash and having unnecessary duplicates in the cache.

Any suggestions?