I opened one of my MRF-350 units to look at the wiring of the IR output jacks. All of the 3.5mm jacks are 3 conductor (tip, ring, sleeve). But, only the RF jacks use the ring, i.e., the RF output has 5v on the ring. The IR output jacks have the ring connected to ground. So, the IR outputs are really mono. Inserting a plug part way into the jack isn't going to get you anything.

In the case of the Sony TV, which appears to use the stereo ring for signal, one has to either wire a cable whereby the URC tip goes to the Sony ring, or you use adapter cables to achieve the same result.