HTTP redirect conserving the payoad in the POST

The easiest thing may just be to issue a 307 redirect, which should (in most browsers) honor the initial request method:

when RULE_INIT { 
    set static::ext_url "http://www.somewhereincloud.mx/uri" 
} 
when HTTP_REQUEST {
    if { [HTTP::method] eq "POST" } { 
        if { [HTTP::header Content-Length] ne "" and [HTTP::header value Content-Length] <= 1048576 } { 
            set content_length [HTTP::header value Content-Length] 
        } else { 
            set content_length 1048576 
        } 
        if { $content_length > 0 } { 
            HTTP::collect $content_length 
        } 
    } 
} 
when HTTP_REQUEST_DATA { 
     See if the ID field is in the POST 
    if { [string tolower [HTTP::payload]] contains "id=" } {
         get the ID value
        if { [URI::query ?[HTTP::payload] id] equals "12345" } {        
             perform 307 redirect         
            HTTP::respond 307 Location $static::ext_url
        }
    }
}