Now the wierd part: this configuration doesn't work if the front-end isn't running. If I attempt to change channels via the command line, MythWeb, or just the back-end changing channels to record a show, the LED blinks but the cable box ignores the input. I have to have the front-end running for any of this to work.
The immediate work-around is to run the front-end in a window, minimized, but this is one of those annoying bugs that I'll obsess about until I have it fixed. Anyone have any ideas?