Here is a way to mirror a pose file. Save a pose file for the character (do NOT include morphs). Open the pz2 in a text editor, you need to do a globall search and replace. You will replace:

k  0  

with k 0 -

Note that there MUST be two blank spaces after the "k" and two blank spaces after the "0" in both the search and replace text. Performing the search and replace will mirror all the positive rotations. The search and replace will also have added an extra minus sign to any negative rotations. Now do another global search and replace of "--" with blank space, this will mirror the negative rotations. Now delete all the "rotateX" channels. Last of all delete all translation channels from the file. You can delete the translation channels manualy whilst in the text editor, it's much easier to do this step in the free utility Mat Pose Edit (version 2.0.3).