Do note that I have not thoroughly tested this method yet, and I don't know how many of you guys know it already.
But hell, it's a nice little trick so I'll post it here.
What You Need
A Humanoid Model (preferably with two arms and two legs XD)
3DS Max 2010 and above (if you're importing a model into brawl)
A set of bones from brawl. or a set of bones in general :l
1: First move around your brawl characters bones to fit inside the model, I'm assuming you've done so already if you're rigging it *shrug*
2: Click the left or right polygon for arms and legs (I don't know if it works on whole meshes yet) and right click the skin modifier AFTER you've finished rigging said arm or leg.
3: Copy that skin modifier using right click/Copy
4: Unclick the polygon you're working on and click the polygon on the opposite leg/arm polygon.
5: Click on the skin modifier for the right/left leg or arm that isn't already rigged
6: Scroll down to the toolbox where all the mirror paremiters are.
7: Find the Mirror paste button and make sure you still have your skin modifier selected, then click Mirror Paste.
And there you go
You may still have to do a little tweeking to get both sides the same(On my character I just needed to fix the foot on the pasted side) but overall it should save quite a bit of time on rigging in general.