Check out
http://www.bsdhome.com/comserv/
We used to have a Minix (small Unix-clone, similar to BSD) with 12 RS232 connectors and it was working as a switch for the CNC's, the only thing the operator had to do was to drop the file to be transmitted in the folder for the specific machine. I can't remember the name of the software but it should be possible to create a program for com port handling pretty easily with Linux with similar functionality.
--Sven