How can I split the 5.1 audio channels from an AC3 file into individuals streams (preferably on a Mac)?

Assuming you have it in an MP4 container... the following should output an MP4 with six different audio streams, each one a mono rip of one of the audio channels:

ffmpeg -i input.mp4 -vn -filter_complex channelsplit output.mp4

This more complicated ffmpeg command will split the AC3 file into several different files:

ffmpeg -i input.mp4 -vn -filter_complex 'channelsplit=channel_layout=5.1[FL][FR][FC][LFE][SL][SR]' -map '[FL]' front_left.ac3 -map '[FR]' front_right.ac3 -map '[FC]' front_center.ac3 -map '[LFE]' lfe.ac3 -map '[SL]' side_left.ac3 -map '[SR]' side_right.ac3

This will only work on builds of ffmpeg compiled with support for filters... but I think most ffmpeg builds should have this.

Audacity has an Export Multiple feature that does exactly that.