...looking for something to sit inline with an RS232 type connection (send wire, rec wire, common) and be able to monitor the communications through the connection?

A bit like a port monitor, but able to access another machine's serial output to an external peripheral device. I can't install a port monitor on the machine itself, so I'm having to monitor the connection remotely.

Hope this makes some sense to someone?

 

How about this?

If that isn't any use, try searching for "Datascope" and RS232.

 

You could just sniff off the rx and tx lines using another PC. Just tee into each line and hook it up to the rx line and ground on the PC's serial port. The monitoring PC will need two serial ports, one to monitor the tx and one to monitor rx. Then just run some kind of terminal emulator program (HyperTerminal, ProComm, etc.) to capture the data.