How can I use an audio device's other channel for input to iChat and Skype?
Well, seems like I just had to think about it a little bit more. My workaround is not possible using only system utilities. I used Soundflower and LineIn from Rogue Amoeba software.

Using LineIn, I could select channels 3-4 specifically from the PreSonus device and route those to the two-channel Soundflower device. Then, I simply selected the Soundflower device as my input in iChat.