iRule to URL rewrite

I guess the answer depends on what the portal is. Is this an APM portal or are you load balancing another portal product with an LTM?

If the latter, once that URL gets to the client it's too late. So you're best option is to use an iRule to catch the responses from the portal and rewrite the www.google.com links. I'd probably use an empty STREAM profile and STREAM iRule. It might look something like this:

when HTTP_REQUEST {
    HTTP::header remove Accept-Encoding
    STREAM::disable
}
when HTTP_RESPONSE {
    if { [HTTP::header Content-Type] contains "text" } {
        STREAM::expression {@www.google.com@www.company.com@}
        STREAM::enable
    }
}