I uploaded a build of the plugin with experimental 5.1 output.
This build doesn't decode digital 5.1 audio that would go through the optical output on the ps2, instead, it tries to decode the ProLogic sound data.
Obviously, to use it, you need a 5.1 speaker set.
As a note, I don't have a 5.1 set in this house, so I had to check it using a pair of stereo speakers for the front L, C and R channels and a pair of headphones for the rear L and R. Meaning I haven't been able to check if the LFE output is correct.