I spent the last several hours learning how twisted the animation bvh format is. *rubs head* Blender does have the ability to flip a pose, but there currently doesn't seem to be a way to import one in a way it can be used in SL. Blender can now import bvh files by default, and there's a method to take blender animations and export them, but the glue in between isn't there.
This takes other bvh files and tries to tune them to work in SL. It's worth reading how it works - there's a lot of details there about what SL expects from animations that I haven't seen talked about elsewhere.