I thought the voltage designations sent to an IR LED might be the same as those sent to a hardwired unit but it sounds like there's no chance that is true.

Right. Infrared signals are modulated pulses, frequencies of infrared light. The IR codes are actually digital numbers embedded in those frequency pulses.

The wired Sony stalk is much more low-tech, it's just a set of continuous resistor values on the wire.

The two are completely incompatible.

I guess I'll mount the LED somewhere then and program with the kenwood IR remote.

Or better yet with the Rio remote (in case the SWI-X supports Press And Hold functions when it learns).