Use another Mac connected via thunderbolt as an audio input source? Basically, what I'm trying to do is use one computer with AU Lab to process some audio (real-time, of course) and use that as an audio source for another computer.

Is it possible to use a thunderbolt-attached Mac as an audio output source or an audio input source, either with or without a third-party application?
I don't see how.

Thunderbolt is basically a mux of DisplayPort and PCIe.  So, unless OS X can "present" as an audio device the way it presents as a disk drive in "target mode" then there really is no way to have another Mac act as an audio device.