I just got the following from JTAC. Pretty much says it all!!!
Problem : DNS forwarders not working.
A moinor problem related to it DNS traceoptions not being logged.
Solution : The DNS forwarders is not supported as of now in SRX. This feature was earlier present but was removed due to various issues.
The DNS internally listens only on 127.0.0.1, which is of no practical use.
DNS forwarding is no longer supported since it was removed along with BIND vesion 8 back in 9.3. We upgraded BIND to 9 but have no plans to bring back DNS forwarding. The DNS forwarder currently in place is used with DNSSEC and is not intended for proxying, which explains why it's not listening on any interfaces other than localhost. The documentation team has been sensitized about this to remove any confusion to this matter. I'm sorry for the inconvenience this has caused.
If you need to have DNS forwarding (as a proxy) implemented in your network, you should go ahead and contact your Juniper account representative to file an enhancement request. Otherwise, let me know if you have further questions or if you're ready to close the case.
As for the traceoptions, the file is saved under /var/named. If the 'file' option is not specified, the default filename is /var/named/named.run. Though I do not think this would be of any help to you, as the feature itself is not supported.