Actually, i suggest resolve the issue by two possible solutions ..
1. Change default port number of syslog. You can view the default configuration (including definition junos-syslog application), using
show groups junos-defaults
but you can not change it however override it, by redefining an application with name "junos-syslog". I have not tested this solution however you can try it and let me know if it rocks ...
2. You can use Destination NAT, i.e. change the port number from 512 to any port which is destined to your external syslog server ... e.g.
set security nat destination pool ext-syslog <SYSLOG IP> destination-port <new port number>
set security nat destination rule-set syslog-nat from interface <source interface>
set security nat destination rule-set syslog-nat rule 1 match destination-address <SYSLOG IP>
set security nat destination rule-set syslog-nat rule 1 match destination-port 512
set security nat destination rule-set syslog-nat rule 1 then destination-nat pool ext-syslog
hope it helps
regards