To close the loop on this, I wired up my second-hand MRX-2 into my system today and connected the RS232 cable between the MRX-2 and the Sony TV's RS232 input port. I then changed the setting on the TV to receive serial (as opposed to HDMI) RS232 control. Unfortunately after experimenting with several cables, the MRX-2 was unable to control the TV at all through the RS232 output. I did not see any way in CCP to configure the RS232 output so it could be that RS232 on the MRX-2 only can be used when the device is set up in a Total Control system.

I had a cable with a mono 3.5mm plug on one end and a (now) useless receptacle on the other end so I decided to cut the mono plug off and connect it to a stereo cable with a 3.5mm stereo plug on one end (because the Sony TV for some reason requires a male stereo plug, while the MRX-2 and other URC devices require a mono plug for IR output). Fortunately I was able to wire this up correctly the first time and it works fine.