There are so many syslog applications out there from free to paid versions. I would know which one could be used for over the WAN.
But I would think this is a security risk. I just implemented a syslog server for our VPN concentrator to log all traffic and logins. The syslogs can reveal alot about your system. A sniffer can easily pick up these details, as I have seen a few articles stating this fact.
Is it possible to create a ipsec tunnel (VPN) between the two networks? This would relieve your security concern and give my more options to choose a typical syslog app. You will be much safer keeping the traffic within the tunnel.