The problem with Tom's second drawing (scarf over post) is that you've reduced the thickness of the beam by one-half at the point of greatest shear stress. As I understand braces, they are designed to prevent racking, not to carry load (though they may contribute to load bearing somewhat). The location of the scarf over the brace isn't so that it is supported by the brace, but rather it is located so that the combination of shear stress and bending stress are as neutral as you can get. It just so happens that it falls about a brace-length's distance off the post...
