I too will have this problem soon but was thinking DIY. I was considering multiplexing the inbuilt serial port ala terminal server. Then it would be a case of special device drivers and some brains in the external unit.

The main problem is a micro with enough serial ports. To have 3 ports in total (including the original which gets used) you'd need a micro with 4 serial ports.

Ethernet based ones are probably a little overkill but would be quite easy to use from custom software. i.e. instead of opening a tty, you open a socket instead.