I tried this from the wiki on my RB450.
Accessing a serial device directly by SSH / telnet / mac-telnet
If you want to access the device directly without logging into RouterOS first, use the special-login command.
First create a new user for access to the serial device:
/user add name=serial password=secret group=read
Then connect this username to the desired serial port:
/special-login add user=serial port=serial0
Now when you connect to your RouterOS machine using SSH (or telnet/mac-telnet) and use the username/password of the user you just created, you will be connected to your serial device directly.
Accessing the port from within RouterOs works with the following:
/system serial-terminal serial0
But ssh to the ip with the serial username and password just drops me to a RouterOs prompt.