Currently there is no way to capture log messages from devices that are connected through the terminal server. There was a wish bug filed to allow each terminal ports messages to be logged to a syslog server. (CSCds08232 - Reverse Telnet logging for async port messages through syslog).
You can however set up each of the 6 routers to point to the syslog server on the central LAN. As long as the routers can ping the server, they can send their syslog messages to it.
