I want to be able to place the malcolm rig in a contact pose, then hit mirror pose and have the left side do what the right side was doing and the right side do what the left was doing but it only affects one side (whichever one is selected) and puts it in the same pose as the other one. It's a pretty useless feature if I still have to pose it manually

We don't have a button for that.
But you can make your own script and make a button for it.

Or you can use animBot, which does have that feature.