I've built a matched set of testing devices that sends an audio signal between tracks via audio-rate automation. The channel turns out to be a little, ahem, noisy, but I'm impressed that it actually works at all. No crash for me, and about 15% of my CPU. Kudos, Ableton devs!
http://synthesist.net/M4L/ParamComm.als.zip
To use the set, drop an audio clip in the "Source" track, and when you start it, it will emerge from the "Dest" track's output. Kind of sounds like Redux + Frequency Shifter for me. You will probably need to refresh the parameter names being used (L and R) as the communication channel. Just reselect them in the menus.
DISCLAIMERS: This is obviously *not* something that you'd do indiscriminately! Turn your sound levels down before trying it!! Makes a good stress test.