Been through exactly this.

You need Logic 5.3 onwards. (Maybe 5.2, but I never saw that one).

Basically the sequence of events is this:

1: Create your audio object, and turn it into an 'audio instrument x' in the 'cha' section of the parameter box.

2: Select your multi-channel VST or Logic instrument (EXS24) in the input box - making sure to select 'multi - channel' instead of mono or stereo on the first level menu.

3: Set up your instrument to use multi-channel output. The latest version of Battery is incompatible with Logic in multi-channel mode, so you might need to go back a version or two.

4: Go back to the environment and create another audio object. Set it to 'aux x' in the cha section.

5: The 'input' box should allow you to select 'instrument x -> output y'

6: Set your AUX object to mono or stereo depending on application.

7: If forced to use stereo channels on the instrument side, you can double them up as mono channels by hard panning in the instrument, then creating mono AUX objects for the left and right channel numbers. Of course, you can pan these as needed on the output bus!

Have fun.

Oh, and remember you can use aux objects to create feedback loops, and put things in the way of delays... Create sends from send chains... Generally f*ck about with the sound. They're fun!

T*