DNS:rrtype undefined procedure

The $rr is undefined in this case. That's more appropriate with an answer with several resource records (and requires a foreach loop as shown in the example code on the rdata wiki page to get the rr variable defined). I think you want something like this instead:

when DNS_REQUEST {
  if { [DNS::question type] eq "TXT" } {
    rateclass dns_rate_shape
  }
}