ffmpeg: stereo to 5.1 with all audio mapped to specific channel (either FL/FC/FR etc.)

I have a stereo audio/video file from youtube that contains a 20-20,000hz test tone sweep. I want to make 6 different versions of this with the same video, but in each case all of the audio mapped to only one channel. That is so that I can play the file on my HTPC and run the separate per-speaker EQ on my AVR and equalize the speakers independently, having the test tone only go to one speaker at a time, depending on which of the test files I select.

So for each of the 6 files, I want the audio mapped to one of these channels, and all of the other channels blank:

Could someone please help me figure out the correct syntax to do that>

thanks!