As PAW13 I did a small addon board to the Bus Pirate. Sniff-A-Lot combines both TX and RX of a serial port into one signal that can be monitored by the Bus Pirate.
This works as long as RX and TX is not transmitting simultaneous (Full Duplex). In most cases the communication is only Half Duplex. where the computer sends a command to the device and the device replies and this gets repeated.
The Sniff-A-Lot handles a both a real RS-232 +/- 12volt communication as well as TTL-level serial comms – both with Idle High and Idle Low versions.