Update: V 1.0 : updated the script to also incorporate user defined channels, now the flipper will work even if your naming conventions are off.
This is the bare bones script you'll need to flip a character pose from left to right. Basically, if you have just Left-hand controls selected, running this script will mirror the pose. If you have left and right side controls selected, this will Flip the pose. It works by storing the transform information of all the selected objects and then assigning them to their opposite objects.
If you're new to scripting then just know that all you have to do is change the words on line 6 and 7 to match up with the way your controllers are named. Replace the names as described, then make this script into a button. You do that by selecting the whole script and middle mouse dragging up to the shelf.
It's simple and good enough for custom rigs, or as a command button in the animSchoolPicker.