Well, you could make a hub with RS-232, unless there is something I'm missing. But first, are you sure you want a hub ( all computers hear all messages, if they are intended for that computer or not ) or do you really want a switch ( 1 communication path between the sender and receiver ) A hub will be simple, a switch is possible, but you would have to create some sort of switching protocol. Maybe there is already something you can use for a protocol, but i wouldn't be aware of it. I'll poke around a little later and see if I can come up with something.
Otherwise, you might think about another simple serial method, RS422, RS485, CAN, SPI or I2C. One of those might already have a portocol to do what you want.