Switching/multiplexing RS232 signal lines
Yes, have a look at these AD analog switches. You can have SPST or SPDT switches, several of them with on resistances less than 1\$\Omega\$.  

edit
I first suggested the ADG849, but Arik correctly points out that this is only limited to 5V operation. I had checked this, but I must have changed my mind and picked a different device at the last moment.  

  

The ADG1436 is a better choice, also because it's a dual switch, so you only need one to switch Rx and Tx. It can operate at \$\pm\$15V, and switch signals to the rails. EIA-232 is \$\pm\$15V maximum, but chances are that you have 12V or, if you use something like a MAX232, only 10V.
The ADG1436 has an \$R_{ON}\$ of 2.6\$\Omega\$, and switches in about 100 nanoseconds. Contrary to a relay, also  hardly requires any power: 0.001\$\mu\$A(!) at 25°C.