Only thing is you cannot switch this thing to send to the aux loop pre-fader. You can do what Scrubs said, it will work fine, just note that you may end up hearing both the dry uncompressed signal from the original input channel AND the comrpessed signal that you return thru the aux loop.
You can test that by setting up the signal path that scrubs describes, then turn the mic channel fader down and see if that also cuts off the signal going out the fx send to the compressor. If not then you will be able to isolate the compressed sound. If it does cut it off, then you will have to pan the dry mic opposite the compressed returned mic in order to isolate the compressed sound by itself.....