Can RouterBoard do a serial-to-serial over IP ?
serial ↔ RB450 ↔ IP Tunnel ↔ RB450 ↔ serial
It can do one side of a TCP session. (server socket, but not server client by the looks of it, but I could be wrong, best to talk to mikrotik support I guess to get the definitive answer)
http://wiki.mikrotik.com/wiki/Serial_Port_Usage#Accessing_a_serial_device_as_if_it_were_physically_connected_to_your_PCs.27_COM_port
Alternatively have a look at, (have had good success using their serial to wifi adaptors)
http://en.usr.cn/index.php?route=product/category&path=9
What is the application? I have some code that we use at work where most serial devices are connected to the production system using a serial to IP script running under linux and windows. Its pretty simple stuff.